drm/i915/gem: Extend mmap support for lmem

Local memory objects are similar to our usual scatterlist, but instead
of using the struct page stored therein, we need to use the
sg->dma_address.

Signed-off-by: Abdiel Janulgue <abdiel.janulgue@linux.intel.com>
Cc: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Matthew Auld <matthew.auld@intel.com>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Link: https://patchwork.freedesktop.org/patch/msgid/20200103204137.2131004-1-chris@chris-wilson.co.uk