drm/i915/display: Fix mode private_flags comparison at atomic_check

drm/i915/display: Fix mode private_flags comparison at atomic_check

This patch fixes the private_flags of mode to be checked and
compared against uapi.mode and not from hw.mode. This helps
properly trigger modeset at boot if desired by driver.

It helps resolve audio_codec initialization issues if display
is connected at boot. Initial discussion on this issue has happened
on below thread:
https://patchwork.freedesktop.org/series/74828/

v2: No functional change. Fixed the Closes tag and added
Maarten's RB.

v3: Added Fixes tag.

Cc: Ville Syrjä <ville.syrjala@linux.intel.com>
Cc: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Cc: Kai Vehmanen <kai.vehmanen@linux.intel.com>
Cc: Souza, Jose <jose.souza@intel.com>
Fixes: 58d124ea2739 ("drm/i915: Complete crtc hw/uapi split, v6.")
Closes: https://gitlab.freedesktop.org/drm/intel/issues/1363
Suggested-by: Ville Syrjä <ville.syrjala@linux.intel.com>
Signed-off-by: Uma Shankar <uma.shankar@intel.com>
Signed-off-by: SweeAun Khor <swee.aun.khor@intel.com>
Reviewed-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20200326125111.11081-1-uma.shankar@intel.com