kms: Skip mode copy for connectors without any modes

kms_screen_probe() unconditionally memcpy()s drmModeModeInfo from
con->modes[0] regardless of con->count_modes. drmModeGetConnector(3)
documents that drmModeConnector::modes is NULL whenever count_modes is
zero, which is normal for any disconnected connector (no EDID, no
mode list).

Currently this only matters in practice when the caller asks for
universal planes via DRM_CLIENT_CAP_UNIVERSAL_PLANES, because that is
when init_cursor()'s kms_device_open() path runs and probes every
connector returned by drmModeGetResources(), including the
disconnected ones.

Concrete reproducer on a Rockchip RK3576 board (driver: rockchip-drm)
with the integrated DisplayPort connector unplugged and HDMI plugged
in. card2 reports two connectors: DP-1 (disconnected, count_modes=0,
modes=NULL) and HDMI-A-1 (connected). With
COG_PLATFORM_DRM_CURSOR=1 set, cog crashes with SIGSEGV inside
kms_screen_probe() at the first iteration of kms_device_probe_screens
on the disconnected DP-1 connector.

Trace from the suspect line, captured by an instrumented build:

    [cursor-debug] kms.c kms_screen_probe: connector id=74 type=10
        connection=2 count_modes=0 modes_ptr=(nil)
    [cursor-debug] kms.c kms_screen_probe: about to memcpy
        con->modes[0] (count_modes=0, modes=(nil))
    *** SIGSEGV ***

Guard the memcpy on count_modes > 0. screen is calloc()'d in
kms_screen_create(), so on the no-modes path screen->mode and
screen->{width,height} stay zeroed -- which is correct: a connector
with no modes has no screen geometry to report.

This is reproducible on any system with a multi-output DRM driver
and at least one connector unplugged at startup; the FlipperOne
hardware just makes it easy to demonstrate.

Author: zhovner

platform/drm/kms.c

@@ -361,9 +361,11 @@ static void kms_screen_probe(struct kms_screen *screen)
     else
         screen->connected = false;
 
-    memcpy(&screen->mode, &con->modes[0], sizeof(drmModeModeInfo));
-    screen->width = screen->mode.hdisplay;
-    screen->height = screen->mode.vdisplay;
+    if (con->count_modes > 0) {
+        memcpy(&screen->mode, &con->modes[0], sizeof(drmModeModeInfo));
+        screen->width = screen->mode.hdisplay;
+        screen->height = screen->mode.vdisplay;
+    }
 
     drmModeFreeConnector(con);
 }