msm: kgsl: Do GX GBIF halt only if GX is on

For GMU fault handling, GX may be off. So do not trigger
GX halt or soft reset if GX is off.

CX halt may still be initiated as GBIF HALT and HALT ACK
registers are powered by CX GDSC.

Change-Id: Ibf00c75802b6c7124821b24ef85076f3e62d4ad2
Signed-off-by: Harshdeep Dhatt <hdhatt@codeaurora.org>