drm/gem-cma-helper: Fix kerneldocs

Use the correct name for the function argument in the docs.

Cc: Noralf Trønnes <noralf@tronnes.org>
Cc: Daniel Vetter <daniel.vetter@ffwll.ch>
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20171114191021.15591-1-ville.syrjala@linux.intel.com
Acked-by: Noralf Trønnes <noralf@tronnes.org>
Reviewed-by: Alex Deucher <alexander.deucher@amd.com>

drivers/gpu/drm/drm_gem_cma_helper.c

@@ -401,7 +401,7 @@ EXPORT_SYMBOL_GPL(drm_gem_cma_get_unmapped_area);
  * drm_gem_cma_print_info() - Print &drm_gem_cma_object info for debugfs
  * @p: DRM printer
  * @indent: Tab indentation level
- * @gem: GEM object
+ * @obj: GEM object
  *
  * This function can be used as the &drm_driver->gem_print_info callback.
  * It prints paddr and vaddr for use in e.g. debugfs output.