drm/i915: Use fb->format->is_yuv for the g4x+ sprite RGB vs. YUV check

g4x+ sprites have an extra cdclk limitation listed for RGB formats.
For some random reason I chose to use cpp>=4 as the check for that.
While that does actually work let's deobfuscate it by checking
for !is_yuv instead. I suspect is_yuv didn't exist way back when
I originally write the code.

Also drop the duplicate comment.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20200206201204.31704-2-ville.syrjala@linux.intel.com


Acked-by: Jani Nikula <jani.nikula@intel.com>