msm: kgsl: Add KGSL_RB_DEVICE macro

The ringbuffer structures are static members of struct adreno_device
which means that they are permanently associated with a specific
adreno device and by extension a struct kgsl_device too. The upshot
is that we can use macro math to derive the adreno device from
a ringbuffer pointer and get rid of the device shortcut in the
ringbuffer struct.  This also gives us a chance to clean up
how functions use the ringbuffer and adreno_device structs
to limit unnessesary dereferencing.

Change-Id: Ic0dedbad909ef71e99cd3319713cee38fb1700f0
Signed-off-by: Jordan Crouse <jcrouse@codeaurora.org>