Documentation/gpu: use code-block with proper language

Now that we don't have automatic syntax highlighting, use the code-block
directive with the explicitly selected language, where appropriate.
Signed-off-by: Jani Nikula <jani.nikula@intel.com>

Documentation/gpu/drm-kms.rst

@@ -215,7 +215,7 @@ Connectors state change detection must be cleanup up with a call to
 Output discovery and initialization example
 -------------------------------------------
 
-::
+.. code-block:: c
 
     void intel_crt_init(struct drm_device *dev)
     {

Documentation/gpu/drm-mm.rst

@@ -45,7 +45,7 @@ the radeon_ttm.c file for an example of usage.
 
 The ttm_global_reference structure is made up of several fields:
 
-::
+.. code-block:: c
 
               struct ttm_global_reference {
                       enum ttm_global_types global_type;