drm/i915: Trim the object sg table

At the moment we allocate enough sg table entries assuming we
will not be able to do any coalescing. But since in practice
we most often can, and more so very effectively, this ends up
wasting a lot of memory.

A simple and effective way of trimming the over-allocated
entries is to copy the table over to a new one allocated to the
exact size.

Experiments on my freshly logged and idle desktop (KDE) showed
that by doing this we can save approximately 1 MiB of RAM, or
when running a typical benchmark like gl_manhattan I have
even seen a 6 MiB saving.

More complicated techniques such as only copying the last used
page and freeing the rest are left to the reader.

v2:
 * Update commit message.
 * Use temporary sg_table on stack. (Chris Wilson)

v3:
 * Commit message update.
 * Comment added.
 * Replace memcpy with copy assignment.
   (Chris Wilson)

Signed-off-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Cc: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Link: http://patchwork.freedesktop.org/patch/msgid/1478704423-7447-1-git-send-email-tvrtko.ursulin@linux.intel.com

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