drm/i915/rkl: Don't try to access transcoder D

drm/i915/rkl: Don't try to access transcoder D

There are a couple places in our driver that loop over transcoders A..D
for gen11+; since RKL only has three pipes/transcoders, this can lead to
unclaimed register reads/writes.  We should add checks for transcoder
existence where appropriate.

v2: Move one transcoder check that wound up in the wrong function after
    conflict resolution.  It belongs in bdw_get_trans_port_sync_config
    rather than bxt_get_dsi_transcoder_state.

v3: Switch loops to use for_each_cpu_transcoder_masked() since this
    iterator already checks the platform's transcoder mask for us.
    (Ville)

Cc: Aditya Swarup <aditya.swarup@intel.com>
Cc: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Matt Roper <matthew.d.roper@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20200603211529.3005059-10-matthew.d.roper@intel.com
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>