msm: kgsl: Minimize RB start function footprint

There is no need for the extra indirection of two functions
that do the same thing. Use one function to do ringbuffer start
and remove and enum while we are at it.

Change-Id: I998dd528041eaa2414abf6b92e8cda8d54520f70
Signed-off-by: Carter Cooper <ccooper@codeaurora.org>