drm: Share the code to compute color plane dimesions

framebuffer_check() has some hand rolled code to compute the color plane
dimensions based on the subsampled information. Let's share the code
between framebuffer_check() and drm_framebuffer_plane_{width,height}().

Cc: Ben Widawsky <ben@bwidawsk.net>
Cc: Jason Ekstrand <jason@jlekstrand.net>
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/20170321181218.10042-2-ville.syrjala@linux.intel.com


Reviewed-by: Ben Widawsky <ben@bwidawsk.net>