msm: kgsl: Clean up the reset and soft reset paths

a6xx added a new function hook and path for doing a GPU reset that fell
back to the default reset path for a610 but a610 purposely skipped
soft reset and the hard reset path is very similar to that used by
GMU/RGMU enabled devices except for one little bit.

Furthermore, if a610 skipped soft reset then there are no a6xx targets
that do soft reset so there is no reason to have an a6xx specific
soft reset path. And further to that, there isn't any reason to have
any a6xx specific functions (like oob) in the generic soft reset path.

Make the a6xx reset support all a6xx targets, remove unused infrastructure
from the rest of the generic code and cleanup the generic functions
to be a little bit easier to understand.

Change-Id: Ic0dedbadd3570fe148a464ff0dc647d91099ae09
Signed-off-by: Jordan Crouse <jcrouse@codeaurora.org>