msm: vidc: Check if devfreq is valid during vote/unvote bus

When msm_v4l2_close() is called after firmware is unloaded, vidc driver
tries to access hfi device pointer in __devfreq_target() that is already
set to NULL during __unload_fw().
This change checks if devfreq is valid during vote/unvote for bus.

Change-Id: If3a037bd33b06892db9759fead98025cce1b9832
Signed-off-by: Sowmya Pandiri <spandiri@codeaurora.org>