msm: kgsl: Allow event groups to define a readtimestamp function

For much of the driver existence we have quietly been overloading
the kgsl_readtimestamp function to report both global and per-context
timestamps.  The short story is that we are going to start tracking
timestamps by both ringbuffer and context and so we need to start having
type specific read functions.  Allow the user to specify a
readtimestamp callback when initalizing a kgsl_gpu_event_group along
with a priv pointer to pass to it.

To make the transition easier switch kgsl_readtimestamp to take an
opaque pointer instead of a context so we can save ourselves a wrapper
function in the most common cases.

Change-Id: Ic0dedbad7cb8b3a687dda9a02353cf59773fcbf7
Signed-off-by: Jordan Crouse <jcrouse@codeaurora.org>