drm/i915: Standardize on 'cpp' for bytes per pixel

We more or less randomly call the "bytes per pixel" value
'cpp', 'bytes_per_pixel', 'pixel_size', or even 'bpp'. Let's just pick
one and stick to it. I've chosen 'cpp'.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1453316739-13296-6-git-send-email-ville.syrjala@linux.intel.com
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>