msm: kgsl: Remove unneeded GMU core checks

If the device has a valid gmu_dev_ops struct defined then we know that
the GMU/RGMU is enabled and we don't need an additional check in the
functions.

Change-Id: Ic0dedbadfa50015459ca254c18a443e2bc2b565b
Signed-off-by: Jordan Crouse <jcrouse@codeaurora.org>