drm/rockchip: Substitute is_yuv_support() 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>
Reviewed-by: Ville Syrjala <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/1531847626-22248-3-git-send-email-ayan.halder@arm.com