msm: kgsl Check NULL return from kgsl_get_device_from_dev

The function kgsl_get_device_from_dev() might return NULL, which
was not checked in a few places. Check the return value wherever
it is called.

Change-Id: I764b23a7b13acd692ce0e1bf4bd040c7e180c7d8
Signed-off-by: Lynus Vaz <lvaz@codeaurora.org>