drm/i915/dpt: Add a modparam to disable DPT via the chicken bit

Add i915.enable_dpt modparam to allow disabling the DPT
usage in hardware via the chicken bit. Useful when debugging
potential DPT issues.

Quickly smoke tested on ADL.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20230320090522.9909-6-ville.syrjala@linux.intel.com


Reviewed-by: Juha-Pekka Heikkila <juhapekka.heikkila@gmail.com>