Mark CRTC get_mixer_width helper API static as it is
not used outside the file.

changes in v4:
	- Patch introduced in the series
changes in v5:
	- Simplify the inline function (Sean)
Signed-off-by: Jeykumar Sankaran <jsanka@codeaurora.org>
Signed-off-by: Sean Paul <seanpaul@chromium.org>
Signed-off-by: Rob Clark <robdclark@gmail.com>