drm/i915: Allow fastboot to fix up the vblank delay

GOP might not agree with our idea of what the vblank delay should be.
Reuse the LRR codepaths to fix that up via a fastset.

The relevant registers aren't actually double buffered so this is a
little bit dodgy. While I've not seen any real issues from frobbing
these live, let's limit this to just the fastboot case (by only
allowing it when old_crtc_state->inherited==true).

Cc: Paz Zcharya <pazz@chromium.org>
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20250116201637.22486-7-ville.syrjala@linux.intel.com


Reviewed-by: Jani Nikula <jani.nikula@intel.com>