drm/gud: Fix endianness in gud_xrgb8888_to_color() helper

DRM formats are defined to be little-endian, unless the
DRM_FORMAT_BIG_ENDIAN flag is set.  Hence when converting from one
format to another, multi-byte pixel values loaded from memory must be
converted from little-endian to host-endian.  Conversely, multi-byte
pixel values written to memory must be converted from host-endian to
little-endian.  Currently only drm_fb_xrgb8888_to_rgb332_line() includes
endianness handling.

Fix gud_xrgb8888_to_color() on big-endian platforms by adding the
missing endianness handling.

Signed-off-by: Geert Uytterhoeven <geert@linux-m68k.org>
Signed-off-by: Noralf Trønnes <noralf@tronnes.org>
Link: https://patchwork.freedesktop.org/patch/msgid/b47589ed5d8ca44e0956684412e3f16f3227f887.1657300532.git.geert@linux-m68k.org