msm: kgsl: Correctly derive DDR stall cycles from GBIF power counters

GBIF has power counters to count DDR stall cycles for each AXI channel
separately unlike VBIF which has single power counter to count the total
number of DDR stall cycles.

This change fixes the DDR stall cycles calculation from GBIF power
counters by considering stall cycles of all AXI channels.

Change-Id: I31d7354ab903009fe8f13eca5c507485b84fcc1d
Signed-off-by: Deepak Kumar <dkumar@codeaurora.org>