i915g: fix implicit fallthrough

Hi all,

First of all, this is my second contribution with mesa. In addition I am not a c/c++ developer so I do not know what I am doing.

as I stated in the issue #4777 (closed) when I compile mesa I got lots of implicit fallthrogh so I just silence the warning using FALLTHROUGH; sentence.

This change compiles for me and it doesn't do any functional changes.

Thanks for reviewing this MR.

Closes: #4777 (closed)

Fixes: 4e861ac4 ("i915g: Add more optimizations")

Fixes: f34fd58e ("i915g: implement unfenced relocs for textures using tiling bits")

Fixes: beaf039f ("i915g: cleanup static state calculation, part 1")