drm/i915/dmc: Move struct intel_dmc to intel_dmc.h

Move struct intel_dmc from i915_drv.h to intel_dmc.h.

v2: Add includes along with moving the struct.

Reviewed-by: José Roberto de Souza <jose.souza@intel.com>
Reviewed-by: Lucas De Marchi <lucas.demarchi@intel.com>
Signed-off-by: Anusha Srivatsa <anusha.srivatsa@intel.com>
Signed-off-by: Lucas De Marchi <lucas.demarchi@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20210526220256.4097-4-anusha.srivatsa@intel.com