drm/i915: Add state readout for DRM infoframe

Added state readout for DRM infoframe and enabled
state validation for DRM infoframe.

v2: Addressed Ville's review comments and dropped the
unused drm infoframe read at intel_hdmi_init.

v3: Removed a redundant platform check as per Ville's
comment.

Signed-off-by: Uma Shankar <uma.shankar@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/1558015817-12025-13-git-send-email-uma.shankar@intel.com