msm: kgsl: Make clear_pending_transactions target specific

Move adreno_clear_pending_transactions to the target specific code.
Besides getting rid of some ADRENO_REG ambiguity and the need for
target specific masks in the gpudev this also provides some clarity
for a6xx since none of the GMU or RGMU targets end up using this
function in the first place so we can not worry about those and
focus on the a5xx/a6xx targets that do need it.

Change-Id: Ic0dedbadbc7927247c84d446664273c6464d07f5
Signed-off-by: Jordan Crouse <jcrouse@codeaurora.org>