drm/i915/gvt: use its own define for gpio

The definition on i915_reg.h is going to change to depend on
dev_priv->gpio_mmio_base being properly initialized. Define our own
macros since init_generic_mmio_info() is called before than
gpio_mmio_base being set.

Cc: intel-gvt-dev@lists.freedesktop.org
Cc: Zhenyu Wang <zhenyuw@linux.intel.com>
Signed-off-by: Lucas De Marchi <lucas.demarchi@intel.com>
Reviewed-by: Zhenyu Wang <zhenyuw@linux.intel.com>
Signed-off-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20180727193647.8639-2-lucas.demarchi@intel.com