msm: kgsl: Return correct timestamp for consumed timestamp reads

adreno_readtimestamp() would return a register value when asked to
get the KGSL_TIMESTAMP_CONSUMED value.  With per-context timestamps
this no longer holds valid since the value in the register could
correspond to a different contexts timestamp.  Return the start of
packet timestamp for the specified context now that the start of
timestamp values are correctly submitted.

CRs-fixed: 382888
Change-Id: Iad56d07eb4a2973a5906d989c96dd117ff902cea
Signed-off-by: Carter Cooper <ccooper@codeaurora.org>