msm: kgsl: Dump GPU registers only when GX is ON

When we get a timeout in waiting for lowest idle level,
dump GX registers, only when GX is ON.

There could be a possibility that oob request to turn ON
GX gets fail, and we try to set state back to SLUMBER.
During this transition, wait_for_lowest_idle will be
called and will eventually time out. This will
trigger debug logs collection and will read GPU
registers, which will not have valid data.

Change-Id: If0420c40a5439683bccaffca2bbbbb874d2ad19e
Signed-off-by: Pankaj Gupta <gpankaj@codeaurora.org>