drm/i915/dmc: move assert_dmc_loaded() to intel_dmc.c

Start localizing DMC register and data access to intel_dmc.c.

Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Reviewed-by: Lucas De Marchi <lucas.demarchi@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/4b96fe56c9c01bc671992dd6fe619638b157878f.1647870374.git.jani.nikula@intel.com