drm/i915: move intel_csr.[ch] under display/

The DMC firmware is about display. Move the handling under display. No
functional changes.

Cc: Ville Syrjälä <ville.syrjala@linux.intel.com>
Acked-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20200211161451.6867-1-jani.nikula@intel.com