msm: kgsl: Add and use KGSL_DEVICE macro

Add a helper macro to convert an adreno_device pointer to a
struct kgsl_device pointer.  This is mostly syntatic sugar
but it makes the code a bit cleaner and it abstracts a bit of
the ugliness away.

Change-Id: Ic0dedbadd97bda3316a58514a5a64757bd4154c7
Signed-off-by: Jordan Crouse <jcrouse@codeaurora.org>