libplacebo: bump minimum dependency to v1.7.0

This lifts the v1.7.0 dependency from the vulkan-specific check out into
the general libplacebo pkgconfig check, a change which is required for
the following refactor of vulkan/display.c into libplacebo/display.c.

A separate dependency for v0.5 is no longer needed, since it's very old
and no longer shipping anywhere (including debian stable). Therefore,
simplify the logic rather than recreating it exactly.