[media] omap_vout: Added check in reqbuf & mmap for buf_size allocation

The usecase where, user allocates small size of buffer
through bootargs (video1_bufsize/video2_bufsize) and later from application
tries to set the format which requires larger buffer size, driver doesn't
check for insufficient buffer size and allows application to map extra buffer.
This leads to kernel crash, when user application tries to access memory
beyond the allocation size.

Added check in both mmap and reqbuf call back function,
and return error if the size of the buffer allocated by user through
bootargs is less than the S_FMT size.

Signed-off-by: Vaibhav Hiremath <hvaibhav@ti.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>