drm/drm_bufs.c: In switch, add break in default case

Added a "break" in the default case of a switch select statement.
GCC complains, although this "break" is not strictly necessary
for the code to work as expected.

Signed-off-by: Fabio M. De Francesco <fmdefrancesco@gmail.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: https://patchwork.freedesktop.org/patch/msgid/20210417161552.6571-1-fmdefrancesco@gmail.com