mali_kbase: Build driver in OUT_DIR

Currently mali_kbase.ko is built in the git directory for the
module resulting in a lot of intermediate files cluttering up
the directory. This change moves the build out to OUT_DIR
instead.

Test: Build, check no intermediate files created, boot to GPU probe
Signed-off-by: Sidath Senanayake <sidaths@google.com>
Change-Id: I5e457cb3793d102007e765f7f5de21916ff9bbfd