i965: Invert image modifier/tiling inference

i965: Invert image modifier/tiling inference

When allocating images, we record a tiling mode and then work backwards
to infer the modifier. Unfortunately this is the wrong way around, since
it is a one:many mapping (e.g. TILING_Y can be plain Y-tiling, or
Y-tiling with CCS).

Invert the mapping, so we record a modifier first and then map this to a
tiling mode.

Reviewed-by: Jason Ekstrand <jason@jlekstrand.net>