drm/xe/guc: Remove i915_regs.h include

i915_regs.h is not needed, particularly in a header file. What is needed
is i915_reg_defs.h for use of _MMIO() and similar macros.

Signed-off-by: Lucas De Marchi <lucas.demarchi@intel.com>
Reviewed-by: Matthew Auld <matthew.auld@intel.com>
Signed-off-by: Rodrigo Vivi <rodrigo.vivi@intel.com>