iommu/vt-d: Remove unused dma_to_mm_pfn function

Remove dma_to_buf_pfn function, which is not used in the codebase.

This was pointed by clang with the following warning:

'dma_to_mm_pfn' [-Wunused-function]
static inline unsigned long dma_to_mm_pfn(unsigned long dma_pfn)
                            ^
https://lore.kernel.org/r/YYhY7GqlrcTZlzuA@fedora

drivers/iommu/intel/iommu.c:136:29: warning: unused function
Signed-off-by: Maíra Canal <maira.canal@usp.br>
Reviewed-by: Nick Desaulniers <ndesaulniers@google.com>
Signed-off-by: Lu Baolu <baolu.lu@linux.intel.com>
Link: https://lore.kernel.org/r/20211217083817.1745419-4-baolu.lu@linux.intel.com
Signed-off-by: Joerg Roedel <jroedel@suse.de>