msm: vidc: Reduce calls to get buffer requirements

Reduce calls to firmware to get buffer requirements.
This call is very expensive causing too much delay
in video startup. Only calling this when necessary.

Change-Id: I01f59b5402e82a79742f257d39cda1c9504e6dd0
Signed-off-by: Vinay Kalia <vkalia@codeaurora.org>