drm/i915/reg: Add/remove some extra blank lines

Add/remove some blank lines to/from i915_reg.h primarily to help the
scripted refactoring coming up, separating unrelated registers and
keeping the comments together.

v2: Also add some extra blank lines

Reviewed-by: Suraj Kandpal <suraj.kandpal@intel.com> # v1
Link: https://lore.kernel.org/r/20250423100213.720585-2-jani.nikula@intel.com


Signed-off-by: Jani Nikula <jani.nikula@intel.com>