drm/i915/bios: be more explicit about discarding iomem address space

Add one explicit discard of __iomem address space qualifier in
validate_vbt(), and respect it otherwise. This adds clarity in the code,
and reduces the sparse warnings from the module to just one.

Quoting Daniel, "The vbt really is plain old memory. Except that it's
reserved in the e820 table as something special and hence treated as io
range by the kernel. But it is memory, hence casting away the __iomem is
imo the right approach."

Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>