drm: Fix kerneldoc for "Returns" section

The blank line between title "Returns:" and detail description is not
allowed, otherwise the title will goes under the description block in
generated .html file after running `make htmldocs`.

There are a few examples for current kerneldoc at [1][2][3].

v2:
- use Link tag with stable URLs

Signed-off-by: renjun wang <renjunw0@foxmail.com>
Link: https://www.kernel.org/doc/html/v6.10/gpu/drm-kms.html#c.drm_crtc_commit_wait # 1
Link: https://www.kernel.org/doc/html/v6.10/gpu/drm-kms.html#c.drm_atomic_get_crtc_state # 2
Link: https://www.kernel.org/doc/html/v6.10/gpu/i915.html#c.i915_vma_pin_fence

 # 3
Reviewed-by: Thomas Zimmermann <tzimmermann@suse.de>
Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Link: https://patchwork.freedesktop.org/patch/msgid/tencent_37A873672B5CD20DECAF99DEDAC5E45C3106@qq.com