agp/intel-gtt: reduce intel-gtt dependencies more

Don't include stuff on behalf of users if they're not strictly necessary
for the header.

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/7bcaa1684587b9b008d3c41468fb40e63c54fbc7.1636977089.git.jani.nikula@intel.com