i915: Re-clang-format and enforce it in CI.

I want to be able to mash the format button at any point when hacking on
this thing instead of doing bespoke whitespace.

Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/25533>

.clang-format-include

@@ -3,6 +3,7 @@
 
 src/**/asahi/**/*
 src/**/panfrost/**/*
+src/gallium/drivers/i915
 src/amd/vulkan/**/*
 src/amd/compiler/**/*
 src/egl/**/*