drm/doc: use proper cross-references for sections

When I originally wrote these docs, I couldn't manage to insert a
cross-reference to a section. Here's how it can be done.
Signed-off-by: Simon Ser <contact@emersion.fr>
Reviewed-by: Jani Nikula <jani.nikula@intel.com>
Acked-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Cc: Pekka Paalanen <pekka.paalanen@collabora.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20230803095734.386761-1-contact@emersion.fr

Documentation/gpu/drm-mm.rst

@@ -517,6 +517,8 @@ DRM Cache Handling and Fast WC memcpy()
 .. kernel-doc:: drivers/gpu/drm/drm_cache.c
    :export:
 
+.. _drm_sync_objects:
+
 DRM Sync Objects
 ===========================
 

include/uapi/drm/drm.h

@@ -673,8 +673,8 @@ struct drm_gem_open {
  * Bitfield of supported PRIME sharing capabilities. See &DRM_PRIME_CAP_IMPORT
  * and &DRM_PRIME_CAP_EXPORT.
  *
- * PRIME buffers are exposed as dma-buf file descriptors. See
- * Documentation/gpu/drm-mm.rst, section "PRIME Buffer Sharing".
+ * PRIME buffers are exposed as dma-buf file descriptors.
+ * See :ref:`prime_buffer_sharing`.
  */
 #define DRM_CAP_PRIME			0x5
 /**
@@ -756,15 +756,14 @@ struct drm_gem_open {
 /**
  * DRM_CAP_SYNCOBJ
  *
- * If set to 1, the driver supports sync objects. See
- * Documentation/gpu/drm-mm.rst, section "DRM Sync Objects".
+ * If set to 1, the driver supports sync objects. See :ref:`drm_sync_objects`.
  */
 #define DRM_CAP_SYNCOBJ		0x13
 /**
  * DRM_CAP_SYNCOBJ_TIMELINE
  *
  * If set to 1, the driver supports timeline operations on sync objects. See
- * Documentation/gpu/drm-mm.rst, section "DRM Sync Objects".
+ * :ref:`drm_sync_objects`.
  */
 #define DRM_CAP_SYNCOBJ_TIMELINE	0x14