msm: kgsl: Pass device pointers to regulator_get()

Passing a device pointer instead of NULL as the first argument
to regulator_get() allows the device to be taken into account
when finding a matching regulator. This removes the need to
specify a unique regulator_name in struct kgsl_pwrctrl.

Change-Id: I6f6c299d5ad39b8b5b24a465ecabab484ea6d18e
Signed-off-by: Matt Wagantall <mattw@codeaurora.org>