drm/i915: fix documentation build after rename

intel_csr.c was moved under display.

Fixes: 06d3ff6e ("drm/i915: move intel_csr.[ch] under display/")
Reported-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20200302170218.16496-1-jani.nikula@intel.com