winsys/radeon: add an interface for contexts

winsys/radeon: add an interface for contexts

Same idea as in libdrm_amdgpu.

A command stream can only be created for a specific context and it's always
submitted to that context.

This will mainly be used by amdgpu and it's required by the GPU reset status
query too.
(radeon only has a basic version of the query and thus doesn't need this)

Reviewed-by: Christian König <christian.koenig@amd.com>