drm/i915: Accept more fixed modes with VRR panels

It seem that when dealing with VRR capable eDP panels we need
to accept fixed modes with variable vblank length (which is what
VRR varies dynamically). Typically the preferred mode seems to be
a non-VRR more (lowish dotclock/refresh rate + short vblank).

We also have examples where it looks like even the hblank length
is a bit different between the preferred mode vs. VRR mode(s).
So let's just accept anything that has matching hdisp+vdisp+flags.

v2: Document that is_alt_drrs_mode() is a subset of is_alt_vrr_mode() (Jani)

Closes: https://gitlab.freedesktop.org/drm/intel/-/issues/125


Reviewed-by: Jani Nikula <jani.nikula@intel.com>
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20220531191844.11313-4-ville.syrjala@linux.intel.com