msm: kgsl: Unmap memory after using it in CFF capture

Unmap memory from kernel space after it is used in CFF capture
function. The kernel mapping is not used frequently and code
sections that use it will always remap it in kernel.

Change-Id: I7909f11de928b5a5c881a3e693fb54aecd681bd3
Signed-off-by: Shubhraprakash Das <sadas@codeaurora.org>