drm: Remove unnecessary test for ARM.

Since arch/arm/include/asm/pgtable.h contains:

#define io_remap_pfn_range(vma,from,pfn,size,prot) \
                remap_pfn_range(vma, from, pfn, size, prot)

there is no point treating ARM as a special case in distinguishing
between remap_pfn_range() and io_remap_pfn_range().

Signed-off-by: Robert P. J. Day <rpjday@crashcourse.ca>
Signed-off-by: Dave Airlie <airlied@redhat.com>