drm/i915: correct the pitch check for NV12 framebuffer

framebuffer for NV12 requires the pitch to the multiplier of 4, instead
of the width. This patch corrects it.

For instance, a 480p video, whose width and pitch are 854 and 896
respectively, is excluded for NV12 plane so far.

Changes since v1:
    - Removed check for NV12 buffer dimensions since additional checks
      are done for viewport size in intel_sprite.c

Signed-off-by: Dongseong Hwang <dongseong.hwang@intel.com>
Signed-off-by: P Raviraj Sitaram <raviraj.p.sitaram@intel.com>
Cc: Chandra Konduru <chandra.konduru@intel.com>
Cc: Vidya Srinivas <vidya.srinivas@intel.com>
Cc: Ville Syrjälä <ville.syrjala@linux.intel.com>
Cc: Juha-Pekka Heikkila <juhapekka.heikkila@gmail.com>
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/1545208152-22658-1-git-send-email-raviraj.p.sitaram@intel.com