drm/i915: Check for unsupported DP link rates during initial commit

Some BIOSes set an unsupported/imprecise DP link rate (for instance on
TGL A stepping). Make sure that we do an encoder recompute and a modeset
in this case.

Cc: Ville Syrjälä <ville.syrjala@linux.intel.com>
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Imre Deak <imre.deak@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20201003001846.1271151-4-imre.deak@intel.com