iommu/vt-d: Move intel_iommu_gfx_mapped to Intel IOMMU header

iommu/vt-d: Move intel_iommu_gfx_mapped to Intel IOMMU header

Static analyzer is not happy about intel_iommu_gfx_mapped declaration:

.../drivers/iommu/intel/iommu.c:364:5: warning: symbol 'intel_iommu_gfx_mapped' was not declared. Should it be static?

Move its declaration to Intel IOMMU header file.

Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Reviewed-by: Lu Baolu <baolu.lu@linux.intel.com>
Link: https://lore.kernel.org/r/20200828161212.71294-1-andriy.shevchenko@linux.intel.com
Signed-off-by: Joerg Roedel <jroedel@suse.de>