msm: kgsl: Add support for the preamble context flag

Userspace will set a flag in the context if preambles are in use. If
they are, we can safely skip save and restore commands for the
context. GMEM save/restore is still required.  To improve performance,
preamble commands are skipped when the context hasn't changed since
the last issueibcmds.

Change-Id: I21bb8996d62651122dbebcf7a79543679109e1f9
Signed-off-by: Vijay Krishnamoorthy <adivarah@codeaurora.org>