drm/i915/display/vrr: Disable VRR in modeset disable path

This patch disables the VRR enable and VRR PUSH
bits in the HW during commit modeset disable sequence.

Thsi disable will happen when the port is disabled
or when the userspace sets VRR prop to false and
requests to disable VRR.

v2:
* Use intel_de_rmw (Jani N)

v3:
* Remove rmw (Ville)

Cc: Ville Syrjälä <ville.syrjala@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/20210122232647.22688-12-manasi.d.navare@intel.com