msm: kgsl: Use a fixed constant for the ringbuffer size

We do not have dynamic ringbuffer sizes so it doesn't make much
sense to carry around the ringbuffer size as a variable.  Use a
#define constant instead and give the compiler a fighting shot at
optimizing math when it can.

Change-Id: Ic0dedbadcd4ae371bc06086a8647703a30020c68
Signed-off-by: Jordan Crouse <jcrouse@codeaurora.org>