drm/i915: Introduce has_buf_trans_select()

Add a small helper to determine if DDI_BUF_CTL uses the
DDI_BUF_TRANS_SELECT field, and whether we have the
accompanying DDI_BUF_TRANS table in the hardware.

Cc: Imre Deak <imre.deak@intel.com>
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20211001130107.1746-3-ville.syrjala@linux.intel.com
Reviewed-by: Imre Deak <imre.deak@intel.com>