msm: kgsl: Return HI offset from kgsl_perfcounter_get

Previously, kgsl_perfcounter_get only returned the offset
of the LO perf counter register. This assumed that the HI
register would always be adjacent to the LO register. With
VBIF 2.0, this assumption has now been broken, so both the
HI and LO register offsets must be returned.

CRs-fixed: 578771
Change-Id: Ie74da5d797e58a143b89a61aba7ebaf1ed42ed5e
Signed-off-by: Kevin Matlage <kmatlage@codeaurora.org>