gallium: rename libpipe_loader to libpipe_loader_dynamic

gallium: rename libpipe_loader to libpipe_loader_dynamic

With the next commits we'll introduce a 'static' version, which will
essentially load the statically linked-in pipe-drivers, rather than the
standalone pipe-$foo.so ones.

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