msm: kgsl: simplify multiple gen_pool support

Clean up some redundant code and prevent gen_pool_free
from being called on the wrong pool for some error
cases in kgsl_mmu_map.

Change-Id: I0d783fbdf393de5d5481bd0270a042671b7d4e6b
Signed-off-by: Jeremy Gebben <jgebben@codeaurora.org>