msm: kgsl: Move hw_isidle to target specific code

Currently, there is a custom hw_isidle function defined for a6xx but the
function pointer is removed in a6xx_start for non GMU/RGMU a6xx targets
which has the effect of falling back to the generic adreno_hw_isidle
function which confused at least two of us before we figured out what
was happening.

Instead of having a generic adreno_hw_isidle function except for a6xx, add
a hw_isidle function hook for all targets and do the GMU check in the
a6xx specific function. This also has the advantage of moving a a540
specific hack into the a5xx code and removes one of two usages of the
gpucore specific busy mask and the other is in soft fault detect.
Arguably we should get rid that too but for now we can just hardcode
the mask in there and remove the busy mask from the gpucore struct.

And finally it looks like adreno_isidle isn't used as a function hook,
so remove it from the device function tables and mark it static.

Change-Id: Ic0dedbad47fa5dee0e5d903a8f9dd9bf7230b759
Signed-off-by: Jordan Crouse <jcrouse@codeaurora.org>