media: venus: mark venc and vdec PM functions as __maybe_unused

Without PM support gcc could warns about unused functions, thus
mark runtime_suspend/resume as __maybe_unused.

Signed-off-by: Stanimir Varbanov <stanimir.varbanov@linaro.org>
Signed-off-by: Hans Verkuil <hans.verkuil@cisco.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>