mesa: sort out PROVIDES for the glvnd case

mesa: sort out PROVIDES for the glvnd case

If mesa is built with GLVND enabled, it doesn't provide GL / GL ES / EGL
libraries directly. Instead it provides two ICD libraries: libEGL_mesa.0
and libGLX_mesa.so.0. Remove virtual provides from the glvnd case
(dropping incorrect virtual/libglx provider while we are at it) and
replace those with runtime providers (to be used by libglvnd in order to
pull corresponding ICDs).

Signed-off-by: Dmitry Baryshkov <dmitry.baryshkov@oss.qualcomm.com>
Signed-off-by: Mathieu Dubois-Briand <mathieu.dubois-briand@bootlin.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>