gallium/vl: add prime support

gallium/vl: add prime support

This fixes the dri2 opening to check if DRI_PRIME is set,
and picks the correct drm device path to open, this along
with a change to libvdpau allows vdpauinfo to work at least,

Martin Peres tested with nouveau, and there seems to be a
further issue with final displaying, it only works sometimes,
but this patch is at least necessary to help debug further.

Signed-off-by: Dave Airlie <airlied@redhat.com>
Cc: mesa-stable@lists.freedesktop.org
Reviewed-by: Christian König <christian.koenig@amd.com>
Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=67283
Tested-by: Armin K. <krejzi@email.com>