drm/i915: Display WA 827

Display WA 827 applies to GEN9 (excluede GLK) and CNL.
Switching the plane format from NV12 to RGB and leaving system idle
results in display underrun and corruption.
WA: Set the bit 15 & bit 19 to 1b in the CLKGATE_DIS_PSL
register for the pipe in which NV12 plane is enabled.

v2: Addressed review comments from Maarten and
Juha-Pekka Heikkila. Added reviewed by from
Juha-Pekka Heikkila.

v3: Rebased the series

Reviewed-by: Juha-Pekka Heikkila <juhapekka.heikkila@gmail.com>
Signed-off-by: Chandra Konduru <chandra.konduru@intel.com>
Signed-off-by: Nabendu Maiti <nabendu.bikash.maiti@intel.com>
Signed-off-by: Vidya Srinivas <vidya.srinivas@intel.com>
Reviewed-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Signed-off-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/1523245273-30264-11-git-send-email-vidya.srinivas@intel.com