r600: Turn 'r600_shader_key' struct into union

r600: Turn 'r600_shader_key' struct into union

This struct was getting a bit crowded, following the lead of
radeonsi, mirror the idea of having sub-structures for each
shader type. Turning 'r600_shader_key' into an union saves
some trivial memory and CPU cycles for the shader keys.

[airlied: drop as_ls, and reorder so larger fields at start.]
Signed-off-by: Edward O'Callaghan <eocallaghan@alterapraxis.com>
Reviewed-by: Marek Olšák <marek.olsak@amd.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>