msm: camera: memmgr: Add error handling get slot failure

Not calling unmap_hw_va if get_slot() fails causes a stale
entry in smmu CB. since get_slot failed, memmgr table
returns error and memmgr, umd doesn't keep info about this
map entry though smmu CB still has this mapping - This
causes any further map calls on the same 'fd' as smmu
return MAP_EXISTS but CSL bookkeeping doesn't have it.

CRs-Fixed: 2788167
Change-Id: Ibb1450f089b9f175ccdbcfb4f5ecc245a5539bd1
Signed-off-by: Pavan Kumar Chilamkurthi <pchilamk@codeaurora.org>