msm: kgsl: Fix stuck issue while booting legacy

Restore offset value of RSCC_OFFSET_LEGACY to fix a error
while booting the legacy RSC.

Fixes: c832a7c9 ("msm: kgsl: Move rscc to a6xx gmu space")
Change-Id: I079ef1108ea5fd244db8f6f759d937640194af07
Signed-off-by: Hui Li <hul@codeaurora.org>