targets: Use a macro to reduce cut and paste in driver setup.

targets: Use a macro to reduce cut and paste in driver setup.

All the replicated prototypes/function bodies obfuscated the interesting
logic of the file: the mapping from driver enable macros to entrypoints we
expose, and the way that the swrast entrypoints are special compared to
the DRM entrypoints.

Reviewed-by: Emil Velikov <emil.velikov@collabora.com>
Reviewed-by: Nicolai Hähnle <nicolai.haehnle@amd.com>