msm: kgsl: Add support for KGSL_CMDBATCH_MARKER

Sometimes the user mode driver assigns a timestamp and goes to all
the work of constructing a command before it discovered it doesn't
need to be executed on the GPU. If that happens the driver can
set the MARKER bit and let the kernel figure how to keep timestamps
moving forward in a linear fashion.

If no commands are ahead of the marker the dispatcher will discard
it in software (and update the timestamps accordingly). If there
are command ahead of it the marker will wait patiently for those
to expire. If new commands come in after the marker the marker
can be dispatched with NOPs in place of the IBs so the timestamp
accounting stays correct.

Change-Id: Ic0dedbada4006e3cf9d4698a419be93b1620d35a
Signed-off-by: Jordan Crouse <jcrouse@codeaurora.org>