gallium/targets: drop link generation for non DRI targets

gallium/targets: drop link generation for non DRI targets

All three (xvmc and omx) do not have an alternative loading
similar to the dri modules. Thus one needs to explicitly install
them in order to use/test them.

v2:
 - Keep vdpau targets, as an equivalent of LIBGL_DRIVERS_PATH
is being worked on.

Signed-off-by: Emil Velikov <emil.l.velikov@gmail.com>
Reviewed-by: Jon TURNEY <jon.turney@dronecode.org.uk>