drm/i915/dmc: s/intel_csr.c/intel_dmc.c and s/intel_csr.h/intel_dmc.h

Finally, rename the header and source file from csr to dmc.

v2: Add file rename in Documentation.
- Place headers in orders. (Jani)

Cc: Jani Nikula <jani.nikula@linux.intel.com>
Signed-off-by: Anusha Srivatsa <anusha.srivatsa@intel.com>
Reviewed-by: Lucas De Marchi <lucas.demarchi@intel.com>
Signed-off-by: Lucas De Marchi <lucas.demarchi@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20210518213444.11420-6-anusha.srivatsa@intel.com