drm/i915: Needs_dmar, not

The reasoning behind our code taking two paths depending upon whether or
not we may have been configured for IOMMU isn't clear to me. It should
always be safe to use the pci mapping functions as they are designed to
abstract the decision we were handling in i915.

Aside from simpler code, removing another member for the intel_gtt
struct is a nice motivation.

I ran this by Chris, and he wasn't concerned about the extra kzalloc,
and memory references vs. page_to_phys calculation in the case without
IOMMU.

v2: Update commit message

v3: Remove needs_dmar addition from Zhenyu upstream

This reverts (and then other stuff)
commit 20652097
Author: Zhenyu Wang <zhenyuw@linux.intel.com>
Date:   Thu Dec 13 23:47:47 2012 +0800

    drm/i915: Fix missed needs_dmar setting

Reviewed-by: Rodrigo Vivi <rodrigo.vivi@gmail.com> (v2)
Cc: Zhenyu Wang <zhenyuw@linux.intel.com>
Signed-off-by: Ben Widawsky <ben@bwidawsk.net>
[danvet: Squash in follow-up fix to remove the bogus hunk which
deleted the dma_mask configuration for gen6+.]
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>