drm/i915/vrr: Use fixed timings for platforms that support VRR

drm/i915/vrr: Use fixed timings for platforms that support VRR

For fixed refresh rate use fixed timings for all platforms that support
VRR. For this add checks to avoid computing and reading VRR for
platforms that do not support VRR.

v2: Avoid touching check for VRR_CTL_FLIP_LINE_EN. (Ville)
v3: Avoid redundant statements in vrr_{compute/get}_config. (Ville)

Signed-off-by: Ankit Nautiyal <ankit.k.nautiyal@intel.com>
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://lore.kernel.org/r/20250324133248.4071909-10-ankit.k.nautiyal@intel.com