drm/omap: Substitute format_is_yuv() with format->is_yuv

drm_format_info table has a field 'is_yuv' to denote if the format
is yuv or not. The driver is expected to use this instead of
having a function for the same purpose.

Signed-off-by: Ayan Kumar halder <ayan.halder@arm.com>
Signed-off-by: Tomi Valkeinen <tomi.valkeinen@ti.com>