minui: drm: Add support for DRM_FORMAT_XRGB8888

* Used for VMSVGA graphics card in VirtualBox and VMware.
* It looks like vmwgfx_kms.c handles 4 formats: ARGB8888, XRGB8888,
  RGB565, and XRGB1555. RGB565 isn't working, minui doesn't handle
  XRGB1555, ARGB8888 works only for VirtualBox, only XRGB8888 works
  for both VirtualBox (7.0.20) and VMware (17.5.2).

Change-Id: I42f5b82254c596c349fcb7dd6d42de7cb9dad3ac