targets/va: Build va driver into libgallium when building with dri

Currently va driver and dri driver are two separate libraries, when
radeonsi is enabled both libraries contain amdgpu winsys. radeonsi
needs shared winsys to ensure sync between OpenGL and VAAPI works.
Build va driver into libgallium to avoid having two instances of amdgpu
winsys.

Cc: mesa-stable
Closes: #11598
Part-of: <!31079>