drm/i915: Populate possible_crtcs correctly

Don't advertize non-exisiting crtcs in the encoder possible_crtcs
bitmask.

Reviewed-by: Dhinakaran Pandiyan <dhinakaran.pandiyan@intel.com>
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190708162048.4286-9-ville.syrjala@linux.intel.com