msm: Camera: Reset base csi address to NULL

Reset base csi address to NULL after unmapping the memory region
to make sure that the csi hardware registers are not programmed until
base address is mapped to fix the data abort in kernel which occurred
randomly during a stability test.

Change-Id: I7cffb23dfd8a560f750bcb27e529cc54dd52aa65
Signed-off-by: Manogna Reddy Alle <cmalle@codeaurora.org>