msm: kgsl: Make sure arguments to FOR_EACH_RINGBUFFER are dereferenced

The FOR_EACH_RINGBUFFER macro loops through all the ringbuffers in
an adreno_device struture.  Make sure that the arguments to the macro
are properly dereferenced before using them.  While we are here, rename
the arguments to keep confusion to a minimum.

Change-Id: Ic0dedbad3c49a7b9764addb30f0a6d7a2cd68cea
Signed-off-by: Jordan Crouse <jcrouse@codeaurora.org>