misc: sgi-gru: Don't disable preemption in GRU driver

Disabling preemption in the GRU driver is unnecessary, and clashes with
sleeping locks in several code paths.  Remove preempt_disable and
preempt_enable from the GRU driver.

Signed-off-by: Dimitri Sivanich <sivanich@hpe.com>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>