drm/i915/vrr: Be more careful with the bits in TRANS_VRR_CTL

drm/i915/vrr: Be more careful with the bits in TRANS_VRR_CTL

On mtl (at least) clearing the guardband bits in the same write
as the enable bit gets cleared seems to cause an immediate FIFO
underrun. Thus is seems that we need to first clear just the
enable bit, then wait for the VRR live status to indicate the
transcoder has exited VRR mode (this step is documented in Bspec
as well), and finally we can clear out the rest of the TRANS_VRR_CTL
for good measure.

I did this without any RMWs in case we want to toggle VRR on/off
via DSB in the future, and as we know DSB can't read registers.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20221202134412.21943-5-ville.syrjala@linux.intel.com
Reviewed-by: Manasi Navare <manasi.d.navare@intel.com>