drm/i915: Use REG_FIELD_PREP() & co. for TRANS_DDI_FUNC_CTL2

Clean up the TRANS_DDI_FUNC_CTL2 programming/readout by
using REG_FIELD_PREP() & co.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20200313164831.5980-6-ville.syrjala@linux.intel.com


Reviewed-by: Manasi Navare <manasi.d.navare@intel.com>