Fix condition for calling set_pixmap_bo in drmmode_xf86crtc_resize

This matches CreateScreenResources_KMS.

Fixes crash when resizing the screen (e.g. using xrandr) with depth <
24.

Bugzilla: https://bugs.freedesktop.org/104914
Acked-by: Alex Deucher <alexander.deucher@amd.com>