msm: kgsl: Correct aperture dword size

Fix a typo where the intended size is the size of
dbg ahb aperture i.e 0x800 dwords and not 0x2000 dwords.

Change-Id: I3d576bfe6fb0686d7f200df47ea21597f372ed80
Signed-off-by: Harshdeep Dhatt <hdhatt@codeaurora.org>