msm: kgsl: Modify readtimestamp to return read status

Modify readtimestamp() to return the read status instead of
timestamp value. An additional parameter, a pointer to
timestamp is passed in and the timestamp is instead
returned at that pointer address. This will catch instances
where an invalid type timestamp read request is
issued or otherwise to catch any error that might
occur.

Change-Id: Ib709466877d3ad6adf3bd826979ddb4ef8696bd4
Signed-off-by: Harsh Vardhan Dwivedi <hdwivedi@codeaurora.org>