drm/vram: remove DRM_VRAM_MM_FILE_OPERATIONS from docs

drm/vram: remove DRM_VRAM_MM_FILE_OPERATIONS from docs

Commit 02f64b2d8605 ("drm/vram: drop DRM_VRAM_MM_FILE_OPERATIONS") dropped
DRM_VRAM_MM_FILE_OPERATIONS in preference for using DEFINE_DRM_GEM_OPS.

However, it was not dropped from the kernel docs.

Use DEFINE_DRM_GEM_OPS in the illustration on how to define a
struct file_operations for such a DRM driver and remove any reference
to DRM_VRAM_MM_FILE_OPERATIONS.

Signed-off-by: Thadeu Lima de Souza Cascardo <cascardo@igalia.com>
Reviewed-by: Thomas Zimmermann <tzimmermann@suse.de>
Fixes: 02f64b2d8605 ("drm/vram: drop DRM_VRAM_MM_FILE_OPERATIONS")
Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Link: https://patch.msgid.link/20260408-drm_gem_vram_helper_docs-v1-1-4d667a768f75@igalia.com