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

Documentation/gpu/i915.rst

@@ -207,10 +207,10 @@ DPIO
 CSR firmware support for DMC
 ----------------------------
 
-.. kernel-doc:: drivers/gpu/drm/i915/intel_csr.c
+.. kernel-doc:: drivers/gpu/drm/i915/display/intel_csr.c
    :doc: csr support for dmc
 
-.. kernel-doc:: drivers/gpu/drm/i915/intel_csr.c
+.. kernel-doc:: drivers/gpu/drm/i915/display/intel_csr.c
    :internal:
 
 Video BIOS Table (VBT)