drm/i915/vrr: Extract intel_vrr_tg_enable()

drm/i915/vrr: Extract intel_vrr_tg_enable()

Extract the VRR timing generator enable into intel_vrr_tg_enable(),
as a counterpart to intel_vrr_tg_disable().

Note that the CMRR part is probably broken, but so are other
things in the CMRR implementation, and thus it is currently
disabled.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20251020185038.4272-14-ville.syrjala@linux.intel.com
Reviewed-by: Ankit Nautiyal <ankit.k.nautiyal@intel.com>