vdpau_x11.h: update stale comment about how libvdpau finds drivers

When this comment was written, it wasn't clear how the library was going to
figure out which back-end driver to load.  Since then, the wrapper has been
updated to make DRI2 that "VDPAU-specific X extension."

Signed-off-by: Aaron Plattner <aplattner@nvidia.com>
Reviewed-by: José Hiram Soltren <jsoltren@nvidia.com>
vdpau_wrapper: remove unused parameter warnings

Signed-off-by: Rémi Denis-Courmont <remid@nvidia.com>
Reviewed-by: Aaron Plattner <aplattner@nvidia.com>
Signed-off-by: Aaron Plattner <aplattner@nvidia.com>
test: do not clobber CFLAGS

Signed-off-by: Rémi Denis-Courmont <remid@nvidia.com>
Reviewed-by: Aaron Plattner <aplattner@nvidia.com>
Signed-off-by: Aaron Plattner <aplattner@nvidia.com>
test: remove assignment-as-truth-value warning

Signed-off-by: Rémi Denis-Courmont <remid@nvidia.com>
Reviewed-by: Aaron Plattner <aplattner@nvidia.com>
Signed-off-by: Aaron Plattner <aplattner@nvidia.com>
test: fix incomplete prototype

Signed-off-by: Rémi Denis-Courmont <remid@nvidia.com>
Reviewed-by: Aaron Plattner <aplattner@nvidia.com>
Signed-off-by: Aaron Plattner <aplattner@nvidia.com>