drm/i915: Reject user modes that don't match fixed mode's refresh rate

When using a panel with a fixed mode we don't change the refresh
rate of the display. Reject any user requested mode which doesn't
match that fixed refresh rate.

Unfortunately when Xorg sees the scaling_mode property on the
connecor it likes to automagically cook up modes whose refresh
rate is a fair bit off from the fixed refresh rate we use. So
we have to give it some extra latitude so that we don't start to
reject all of it.

v2: sDVO now uses intel_panel_compute_config() too
v3: Add a debug message to inform the user what happened

References: https://gitlab.freedesktop.org/drm/intel/-/issues/2939
References: https://gitlab.freedesktop.org/drm/intel/-/issues/3969


Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Reviewed-by: Jani Nikula <jani.nikula@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20210929184536.8332-1-ville.syrjala@linux.intel.com