avcodec/vdpau: Re-factor pre-hwaccel helper functions into separate header

h264.h and hevc.h are mutually exclusive due to defining some of the same
names. As such, we need to avoid forcing h264.h to be included if we want
hevc decode acceleration to be possible.

However, some of the pre-hwaccel helper functions need h264.h. To avoid
messy collisions, let's move the declaration of all those helpers to
a separate header which we will exclude for the hevc support (which will
be hwaccel-only).

Signed-off-by: Philip Langdale <philipl@overt.org>