drm/i915/gvt: Remove unnecessary check in reg_is_mmio

drm/i915/gvt: Remove unnecessary check in reg_is_mmio

The reg >= 0 check in reg_is_mmio is unnecessary because reg is always
greater than zero in all current use cases.  This is obvious when
checking 'offset' by itself (as offset is defined as an unsigned
integer), but it's also true for the offset + bytes - 1 use case in
intel_vgpu_emulate_mmio_read because bytes > 0.

Signed-off-by: Jonathan Cavitt <jonathan.cavitt@intel.com>
Reviewed-by: Andi Shyti <andi.shyti@linux.intel.com>
Reviewed-by: Zhenyu Wang <zhenyuw.linux@gmail.com>
Signed-off-by: Andi Shyti <andi.shyti@linux.intel.com>
Link: https://lore.kernel.org/r/20250916174317.76521-5-jonathan.cavitt@intel.com