msm: kgsl: Take a reference count on the active adreno draw context

Take a reference count on the currently active draw context to keep
it from going away while we are maintaining a pointer to it in the
adreno device.

Change-Id: Ic0dedbade8c09ecacf822e9a3c5fbaf6e017ec0c
Signed-off-by: Jordan Crouse <jcrouse@codeaurora.org>