drm/i915: Fix lock order reversal in GTT pwrite path.

Since the pagefault path determines that the lock order we use has to be
mmap_sem -> struct_mutex, we can't allow page faults to occur while the
struct_mutex is held.  To fix this in pwrite, we first try optimistically to
see if we can copy from user without faulting.  If it fails, fall back to
using get_user_pages to pin the user's memory, and map those pages
atomically when copying it to the GPU.

Signed-off-by: Eric Anholt <eric@anholt.net>
Reviewed-by: Jesse Barnes <jbarnes@virtuousgeek.org>