drm/i915/skl: Avoid using un-initialized bits_per_pixel

Don't rely on fb->bits_per_pixel as intel_framebuffer_init is not
filling bits_per_pixel field of fb-struct for YUV pixel format.
This leads to divide by zero error during watermark calculation.

Signed-off-by: Kumar, Mahesh <mahesh1.kumar@intel.com>
Cc: Konduru, Chandra <chandra.konduru@intel.com>
Reviewed-by: Sonika Jindal <sonika.jindal@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>