i965/miptree: Add space to store the clear value in the aux surface.

i965/miptree: Add space to store the clear value in the aux surface.

Similarly to vulkan where we store the clear value in the aux surface,
we can do the same in GL.

v2: Remove unneeded extra function.
v3: Use clear_value_state_size instead of clear_value_size.
v4:
 - rename to clear_color_state_size
 - store clear_color_bo and clear_color_offset in the aux buf struct
v5: Unreference clear color bo (Jordan)

Signed-off-by: Rafael Antognolli <rafael.antognolli@intel.com>
Reviewed-by: Jason Ekstrand <jason@jlekstrand.net>
Reviewed-by: Jordan Justen <jordan.l.justen@intel.com>