drm/i915/d12+: Disable DMC handlers during loading/disabling the firmware

Disable the DMC event handlers before loading the firmware and after
uninitializing the display, to make sure the firmware is inactive. This
matches the Bspec "Sequences for Display C5 and C6" page for GEN12+.

Add a TODO comment for doing the same on pre-GEN12 platforms.

v2:
- Add a macro for the number of event handlers.
- Disable the event handlers only on GEN12+.
- s/dev_priv/i915/ in docbook comment.

Signed-off-by: Imre Deak <imre.deak@intel.com>
Reviewed-by: Uma Shankar <uma.shankar@intel.com> #v1
Link: https://patchwork.freedesktop.org/patch/msgid/20220728081440.1676857-1-imre.deak@intel.com