media: rpivid: Remove the need to have num_entry_points set

VAAPI H265 has num entry points but never sets it. Allow a VAAPI
shim to work without requiring rewriting the VAAPI driver.
num_entry_points can be calculated from the slice_segment_addr
of the next slice so delay processing until we have that.

Also includes some minor cosmetics.

Signed-off-by: John Cox <jc@kynesim.co.uk>