targets/radeonsi/omx: convert to static/shared pipe-drivers

targets/radeonsi/omx: convert to static/shared pipe-drivers

The radeonsi counterpart of previous commit - now libomx-radeonsi is
built into the libomx-mesa library. Providing a single library per API.

v2: Include the radeon winsys only when there is a user for it.
v3: Correcly include the winsys. Now with extra brown bag :\

Note: Make sure to rebuild the .omxregister file, by executing
   $ omxregister-bellagio

This patch concludes the unification. Now libomx-mesa will be used
for all hardware - r600, radeonsi and nouveau.

Cc: Leo Liu <leo.liu@amd.com>
Cc: Christian König <christian.koenig@amd.com>
Signed-off-by: Emil Velikov <emil.l.velikov@gmail.com>
Reviewed-by: Christian König <christian.koenig@amd.com>