msm: kgsl: consolidate preamble based context switch code

Move logic for handling preamble based context switch
to the core adreno code. This makes it less burdensome
to implement support for newer GPU families that won't
ever support legacy context switching.

Change-Id: Id9ad5936ff91dcdbc9de869baf0d0b9fcf1b5170
Signed-off-by: Jeremy Gebben <jgebben@codeaurora.org>