drm/xe: Remove dependency on i915_reg.h

Copy the macros used by xe in i915_reg.h to regs/xe_regs.h. A minimal
cleanup is done while copying so they adhere minimally to the coding
style.  Further reordering and cleaning is left for later.

Signed-off-by: Lucas De Marchi <lucas.demarchi@intel.com>
Reviewed-by: Matt Roper <matthew.d.roper@intel.com>
Signed-off-by: Rodrigo Vivi <rodrigo.vivi@intel.com>