msm: kgsl: Replace device->parentdev with device->pdev

The code spends a lot of time converting device->parentdev
from a device pointer to a platform_device and then back into
a device again. Cut through the middle man and track just the
platform_device pointer in kgsl_device.

Change-Id: Ic0dedbada14e2adaee754bbfedc4ca33087c446c
Signed-off-by: Jordan Crouse <jcrouse@codeaurora.org>