drm/msm: Remove ringbuffer restriction from idle

Remove the check in a5xx_idle() to see if idle is being called
on the current ring. a5xx_idle() is to used make sure the
commands in the specified ring have completed and it is entirely
reasonably to expect that a ring switch might need to be
executed first.

Change-Id: Ic0dedbadf45c7f624093efae72f3a9d75dc6857d
Signed-off-by: Jordan Crouse <jcrouse@codeaurora.org>