drm/i915: add support for specifying DMC firmware override by module param

Use i915.dmc_firmware_path to override default firmware for the platform
and bypassing version checks.

v2: add missing param struct member declaration (David)

Tested-by: David Weinehall <david.weinehall@linux.intel.com>
Reviewed-by: David Weinehall <david.weinehall@linux.intel.com>
Cc: Anusha Srivatsa <anusha.srivatsa@intel.com>
Cc: David Weinehall <david.weinehall@linux.intel.com>
Acked-by: Anusha Srivatsa <anusha.srivatsa@intel.com>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20180424122016.2416-1-jani.nikula@intel.com