pipe-loader: wire up the 'static' drm pipe-loader

pipe-loader: wire up the 'static' drm pipe-loader

Add a list of driver descriptors and select one from the list, during
probe time.

As we'll need to have all the driver pipe_foo_screen_create() functions
provided externally (i.e. from another static lib) we need a separate
(non-inline) drm_helper, which contains the function declarations.

v2: rebase on top of virgl support.

Signed-off-by: Emil Velikov <emil.l.velikov@gmail.com>
Acked-by: Rob Clark <robclark@freedesktop.org>