msm: kgsl: Update the RPTR value in the RB snapshot header

Update the RPTR as read from the register into the RB snapshot
header. Without this, a few IBs were not seen in the snapshot dump.

Change-Id: Ic47d3283e5b63515fdea8ba2e1003ce8b24baff7
Signed-off-by: Lynus Vaz <lvaz@codeaurora.org>