msm: kgsl: Remove IOCTL_KGSL_DEVICE_REGREAD from msm_kgsl.h

The code for IOCTL_KGSL_DEVICE_REGREAD was removed long ago
but the header was never cleaned up.

Change-Id: Id3eaf0486fae133dfd72a56ea915f63fcb3f6bb1
Signed-off-by: Harsh Vardhan Dwivedi <hdwivedi@codeaurora.org>