configure: ask vdpau.pc for the default location of the vdpau drivers

configure: ask vdpau.pc for the default location of the vdpau drivers

Rather than using hardcoded values honor the value set at libvdpau
build time - i.e. the moduledir variable from vdpau.pc

Update the omx description to match reality while we're here.

Cc: Christian König <deathsimple@vodafone.de>
Cc: Alexandre Demers <alexandre.f.demers@gmail.com>
Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=80615
Signed-off-by: Emil Velikov <emil.l.velikov@gmail.com>
Reviewed-by: Matt Turner <mattst88@gmail.com>