msm: kgsl: Add a missing field in the a4xx_perfcounters_vbif_pwr

The usercount field for adreno_perfcount_register was missing from
the a4xx_perfcounters_vbif_pwr causing us to read an incorrect
offset when trying to read those registers.

Change-Id: I46a8e407caf4f996f86f52e43eed3d1f106a5741
Signed-off-by: Shrenuj Bansal <shrenujb@codeaurora.org>