drm/i915/vrr: Hide the ICL/TGL intel_vrr_flipline_offset() mangling better

ICL/TGL VRR hardware won't allow us to program flipline==vmin. If we do
that the actual effect will be the same as if we had programmed
flipline=vmin+1, which would make the minimum vtotal one scanline taller
than expected.

To compensate for this we reduce vmin by one, and then program
flipline=vmin+1. So we end up with a flipline value that matches
the expected minimum vtotal. Currently this adjustment happens
in intel_vrr_compute_config() which means that crtc_state->vrr.vmin
will no longer be directly usable for the remainder of the high
level VRR code. That is annoying at best, fragile at worst.

Hide the adjustment in low level code instead. This will allow most
of the higher level VRR code to remain blissfully ignorant about this
fact. Afterwards crtc_state->vrr.{vmin,flipline} will be equal
and match the minimum vtotal, exactly how things already work
on ADL+.

The only slight downside is that the actual register value will no
longer match crtc_state->vrr.vmin on ICL/TGL, but that may already
be the case on TGL because the register value will also have been
adjusted by the SCL.

Note that we must change the guardband calculation to account
for intel_vrr_extra_vblank_delay() explicitly. Previously that
was accidentally handled by the earlier vmin reduction by
intel_vrr_flipline_offset().

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20250918232226.25295-2-ville.syrjala@linux.intel.com
Reviewed-by: Ankit Nautiyal <ankit.k.nautiyal@intel.com>

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