drm/i915: include intel-gtt.h only where needed

Only intel_gt.c and intel_ggtt.c need the interface.

Cc: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Acked-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Acked-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Reviewed-by: Lucas De Marchi <lucas.demarchi@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/034f57db24d6936ac2e4e6830261d791240cdd79.1636977089.git.jani.nikula@intel.com