r600g: suballocate memory for fetch shaders from a large buffer

r600g: suballocate memory for fetch shaders from a large buffer

Fetch shaders are usually destroyed at the context destruction by the state
tracker, so we can put them all in a large buffer without wasting memory.

This reduces the number of relocations sent to the kernel a little bit.

Tested-by: Aaron Watry <awatry@gmail.com>
Reviewed-by: Alex Deucher <alexander.deucher@amd.com>