Revert "msm: kgsl: consolidate preamble based context switch code"

This reverts commit fc5f407f5689dcca3d2d567ffc4f2056b1f41dbd.

Conflicts:
	drivers/gpu/msm/adreno_a2xx.c
	drivers/gpu/msm/adreno_a3xx.c
	drivers/gpu/msm/adreno_a4xx.c
	drivers/gpu/msm/adreno_drawctxt.c