mdss: display: unmap 'fb_addr_va' when the mapping is no longer needed

Remove the mapping to the original continuous splash frame buffer
once its contents have been copied. This avoids a virtual address
leak, and holding a mapping to memory that may be eventually
reclaimed for use by other subsystem.

Change-Id: I8d7b6d954809a74d5a1ae60491e295b95458a2b7
Signed-off-by: Matt Wagantall <mattw@codeaurora.org>