drm/i915: Limit the ioremap of the PCI bar to the registers

In the future we may like to experiment with using a WC map of the GTT
portion. However, that will conflict with i915.ko mapping the entire bar
as UC in order to access the GPU registers. Instead we can shrink the
register ioremap to only map the register block.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Tested-by (IVB): Ben Widawsky <ben@bwidawsk.net>
Acked-by: Ben Widawsky <ben@bwidawsk.net>
[danvet: Squashed-in follow-up fix for gen2/3 registers file size from
Chris Wilson.]
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>