drm/i915: fix port checks for MST support on gen >= 11

drm/i915: fix port checks for MST support on gen >= 11

[ Upstream commit 10d987fd1b7baceaafa78d805e71427ab735b4e4 ]

Both Ice Lake and Elkhart Lake (gen 11) support MST on all external
connections except DDI A. Tiger Lake (gen 12) supports on all external
connections.

Move the check to happen inside intel_dp_mst_encoder_init() and add
specific platform checks.

v2: Replace != with == checks for ports on gen < 11 (Ville)

Signed-off-by: Lucas De Marchi <lucas.demarchi@intel.com>
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20191015164029.18431-3-lucas.demarchi@intel.com
Signed-off-by: Sasha Levin <sashal@kernel.org>