msm: kgsl: Modify kgsl_core_exit to release resources carefully.

kgsl_core_exit() should not call kgsl_sharedmem_uninit_sysfs() and
device_unregister() if those resources have not been allocated.
We check this by seeing if kgsl_driver.virtdev has been
populated or not.

CRs-fixed: 359477
Change-Id: Ic040667b29bd4e006e6f29a50809d94a2bb4b469
Signed-off-by: Harsh Vardhan Dwivedi <hdwivedi@codeaurora.org>