drm/i915/dp: Do not set master_trans bit in bitmak if INVALID_TRANSCODER

In the port sync mode, for the master crtc, the master_transcoder is INVALID.
In that case since its value is -1, do not set the bit in the bitmask.

Cc: Ville Syrjälä <ville.syrjala@linux.intel.com>
Fixes: d0eed154 ("drm/i915: Fix post-fastset modeset check for port sync")
Signed-off-by: Manasi Navare <manasi.d.navare@intel.com>
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20200123002415.31478-1-manasi.d.navare@intel.com