gallium: introduce load_pipe_screen()

gallium: introduce load_pipe_screen()

Introduce load_pipe_screen() public entry point for other code which
dlopen()'s gralloc_dri.so for purposes of loading a pipe_screen. This way
drm_gralloc can avoid static linking of each gallium winsys and driver,
and avoid duplicated logic to figure out which pipe driver to load.

This is based on Rob Clark's work. I moved it into pipe_loader which seems
to be a better spot.

Signed-off-by: Rob Herring <robh@kernel.org>