gallium/drm: Deduplicate screen creation for the dynamic (clover) pipe loader.

gallium/drm: Deduplicate screen creation for the dynamic (clover) pipe loader.

We can just reuse drm_helper.h, which has either the real code or the stub
for all pipe_screens based on the GALLIUM_* driver defines, and the
dynamic pipe loader's .c build will only define one GALLIUM_* driver
define.  The remaining stubs should get GCed by the linker.

Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/6751>