drm/i915/dmc: Let's abstract the dmc path.

Although this abstraction removes the convenience of grepping
for the file name, it:
- makes addition easier.
- makes it easier to tweak global path when experiments are needed.
- get in sync with guc/huc, without getting overly abstracted.
- allows future junction with CSR_VERSION for simplicity.
- Enforces dmc file will never change this standard.

v2: define DMC_PATH inside .c (Lucas)

Cc: Fei Yang <fei.yang@intel.com>
Cc: Jani Nikula <jani.nikula@intel.com>
Cc: Lucas De Marchi <lucas.demarchi@intel.com>
Signed-off-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Reviewed-by: José Roberto de Souza <jose.souza@intel.com> #v1
Reviewed-by: Jani Nikula <jani.nikula@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20210421094406.2017733-1-rodrigo.vivi@intel.com