i965/gen8+: Don't upload the MCS buffer for single-sampled textures

i965/gen8+: Don't upload the MCS buffer for single-sampled textures

For single-sampled textures the MCS buffer is only used to implement
fast clears. However the surface always needs to be resolved before
being used as a texture anyway so the the MCS buffer doesn't actually
achieve anything. This is important for Gen9 because in that case SRGB
surfaces are not supported for fast clears and we don't want the
hardware to see the MCS buffer in that case.

Reviewed-by: Topi Pohjolainen <topi.pohjolainen@intel.com>