drm/i915: Enable a PIPEDMC whenever its corresponding pipe is enabled

Make sure that PIPEDMCs are enabled whenever the corresponding pipe is
enabled.

This is required at least by the latest ADLP v2.18 firmware, which adds
a new handler enabled by default and running whenever the pipe is
enabled at the vertical referesh rate.

Bspec: 50344, 67620

Signed-off-by: Imre Deak <imre.deak@intel.com>
Reviewed-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Tested-by: Gustavo Sousa <gustavo.sousa@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20230102183324.862279-1-imre.deak@intel.com