msm: kgsl: Add ringbuffer priority levels

Add priority levels to ringbuffers and since there are
multiple ringbuffers, place the index of memstore memory
for these ringbuffers after the indexes of contexts. Context id
0 is used to refer to ringbuffer memstore but since there are
multiple ringbuffers the priority level is also used to decide
the memstore index for a ringbuffer. This leaves index 0 of
memstore unused for now.

Change-Id: I922b98dedeb0392cc81095efc100d8c585ab5c17
Signed-off-by: Shubhraprakash Das <sadas@codeaurora.org>