common: Introduce reference counter for mapping physical memory

This is necessary, since when gst_video_frame_map() maps a GstBuffer which
contains physical memory, the phys mem block may be mapped several times
(once per plane, so, for planar formats like I420, three times).

Signed-off-by: Carlos Rafael Giani <dv@pseudoterminal.org>
common: Remove cpu_addr

This is a VPU specific value, which can be stored in the "internal"
member just fine.

Signed-off-by: Carlos Rafael Giani <dv@pseudoterminal.org>