drm/i915/mst: Do not hardcoded the crtcs that encoder can connect

drm/i915/mst: Do not hardcoded the crtcs that encoder can connect

Tiger Lake has up to 4 pipes so the mask would need to be 0xf instead of
0x7. Do not hardcode the mask so it allows the fake MST encoders to
connect to all pipes no matter how many the platform has.

Iterating over all pipes to keep consistent with intel_ddi_init().

Initialy this patch was replaced by commit 4eaceea3a00f ("drm/i915:
Fix DP-MST crtc_mask") but userspace it not correctly using
encoder.possible_crtcs and it was reverted by
commit e838bfa8e170 ("Revert "drm/i915: Fix DP-MST crtc_mask"")

Userspace should be fixed but it might take a while, so bringing this
patch back for now.

Cc: Lucas De Marchi <lucas.demarchi@intel.com>
Cc: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: José Roberto de Souza <jose.souza@intel.com>
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/20190904230241.20638-2-jose.souza@intel.com