Move the checks to decide whether a fastset is possible during the
initial commit to an encoder hook. This check is really encoder specific
and the next patch will also require this adding a DP encoder specific
check.

v2: Fix negated condition in gen11_dsi_initial_fastset_check().
v3: Make sure to call the hook for all encoders on the crtc. (Ville)

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/20201005215311.1475666-1-imre.deak@intel.com