k_mem_pool_malloc reserves some space for block descriptor at the start
of the data block, causing misalignement of returned video buffer.
In our context we want to return a video buffer with aligment matching
the video buffer poll alignment config. Fix that by using the simpler
k_mem_pool_alloc function.
Signed-off-by: Loic Poulain <loic.poulain@linaro.org>