drm/i915/dmc: Step away from symbolic links

Load specific firmware versions for the DMC instead of using symbolic
links. The currently recommended versions are: SKL 1.26, KBL 1.01 and
BXT 1.07.

Certain DMC versions need workarounds in the driver which forces us to
have a tight dependency between firmware and driver. In order to be able
to provide a tested and known working configuration we must lock down on
a specific DMC firmware version.

Cc: Rodrigo Vivi <rodrigo.vivi@intel.com>
Cc: Imre Deak <imre.deak@intel.com>
Cc: Mika Kuoppala <mika.kuoppala@linux.intel.com>
Signed-off-by: Patrik Jakobsson <patrik.jakobsson@linux.intel.com>
Acked-by: Jani Nikula <jani.nikula@intel.com>
Reviewed-by: Mika Kuoppala <mika.kuoppala@intel.com>
Acked-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Signed-off-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1463391057-32350-1-git-send-email-patrik.jakobsson@linux.intel.com