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>

drivers/gpu/drm/i915/intel_pm.c

@@ -3170,7 +3170,8 @@ static void skl_compute_wm_pipe_parameters(struct drm_crtc *crtc,
 		if (fb) {
 			p->plane[0].enabled = true;
 			p->plane[0].bytes_per_pixel = fb->pixel_format == DRM_FORMAT_NV12 ?
-				drm_format_plane_cpp(fb->pixel_format, 1) : fb->bits_per_pixel / 8;
+				drm_format_plane_cpp(fb->pixel_format, 1) :
+				drm_format_plane_cpp(fb->pixel_format, 0);
 			p->plane[0].y_bytes_per_pixel = fb->pixel_format == DRM_FORMAT_NV12 ?
 				drm_format_plane_cpp(fb->pixel_format, 0) : 0;
 			p->plane[0].tiling = fb->modifier[0];