msm: kgsl: Move soft reset to common code

Soft reset on all existing GPU cores is same register write. Remove
soft reset as a core specific function and make it a common function
instead.

Change-Id: Iee97d55da8bded78cabbddb9d191585089e53fc5
Signed-off-by: Tarun Karra <tkarra@codeaurora.org>