msm: kgsl: Check if GX is on before adding regs

The GMU GX registers require the GX domain to be
on in order to be read. Separate these registers
from the rest of the GMU registers in snapshot
and only add them after checking if GX is on.

CRs-Fixed: 2062271
Change-Id: Ib5db9f81ed39757d23510edb4ad93f910c14a4dc
Signed-off-by: George Shen <sqiao@codeaurora.org>