msm: vidc: Align secure buffer mappings to 1MB

Secure buffer mappings should be aligned to 1MB
for the mappings to succeed. This change takes
care of that.

Change-Id: I963df71271b9b24191ac94203d36a25fec90cdbf
Signed-off-by: Vinay Kalia <vkalia@codeaurora.org>