default-providers: fix GLVND providers to be selected at runtime

default-providers: fix GLVND providers to be selected at runtime

The virtual-{libegl,libglx}-icd names are RPROVIDES, not PROVIDES,
because they are a runtime dependency of libglvnd. Make
default-providers follow that and specify PREFERRED_RPROVIDER instead of
PREFERRED_PROVIDER for those names.

Fixes: 9d3b4c9bc403 ("mesa: sort out PROVIDES for the glvnd case")
Signed-off-by: Dmitry Baryshkov <dmitry.baryshkov@oss.qualcomm.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>