radeonsi: add driconf option to enable 2x2 coarse shading (VRS)
The 2x2 coarse shading is enabled if the driconf option is enabled, gl_Position.w != 1, and the pixel shader doesn't use discard/demote.
The 2x2 coarse shading is enabled if the driconf option is enabled, gl_Position.w != 1, and the pixel shader doesn't use discard/demote.