drm/i915/display/dmc: Set DC_STATE_DEBUG_MASK_CORES after firmware load

Specification asks for DC_STATE_DEBUG_MASK_CORES to be set for all
platforms that supports DMC, not only for geminilake and broxton.

While at is also taking the oportunity to simply the code.

BSpec: 7402
BSpec: 49436
Reviewed-by: Imre Deak <imre.deak@intel.com>
Cc: Imre Deak <imre.deak@intel.com>
Cc: Gwan-gyeong Mun <gwan-gyeong.mun@intel.com>
Signed-off-by: José Roberto de Souza <jose.souza@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20210922215242.66683-1-jose.souza@intel.com