msm: kgsl: embed kgsl_context struct in adreno_context struct

Having a separate allocated struct for the device specific context
makes ownership unclear, which could lead to reference counting
problems or invalid pointers. Also, duplicate members were
starting to appear in adreno_context because there wasn't a safe
way to reach the kgsl_context from some parts of the adreno code.
This can now be done via container_of().

This change alters the lifecycle of the context->id, which is
now freed when the context reference count hits zero rather
than in kgsl_context_detach().

It also changes the context creation and destruction sequence.
The device specific code must allocate a structure containing
a struct kgsl_context and passes a pointer it to kgsl_init_context()
before doing any device specific initialization. There is also a
separate drawctxt_detach() callback for doing device specific
cleanup. This is separate from freeing memory, which is done
by the drawctxt_destroy() callback.

Change-Id: I7d238476a3bfec98fd8dbc28971cf3187a81dac2
Signed-off-by: Jeremy Gebben <jgebben@codeaurora.org>