drm/i915: move i915_fixed.h to display/intel_fixed.h

drm/i915: move i915_fixed.h to display/intel_fixed.h

All the users are in display, move the fixed point header under
display. We could also consider making these more general purpose
things, but that takes a bunch more effort. This allows the immediate
cleanup of xe compat i915_fixed.h.

Cc: Lucas De Marchi <lucas.demarchi@intel.com>
Reviewed-by: Lucas De Marchi <lucas.demarchi@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/320c451e116c7807e544a50c67ba79b087a4f218.1712345787.git.jani.nikula@intel.com
Signed-off-by: Jani Nikula <jani.nikula@intel.com>