drm/i915/display: Add HW state readout for VRR

This functions gets the VRR config from the VRR registers
to match the crtc state variables for VRR.

v2:
* Rebase (Manasi)
* Use HAS_VRR (Jani N)

v3:
* Get pipeline_full, flipline (Ville)

Cc: Jani Nikula <jani.nikula@linux.intel.com>
Signed-off-by: Manasi Navare <manasi.d.navare@intel.com>
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20210122232647.22688-14-manasi.d.navare@intel.com