drm/i915: Support for pread/pwrite from/to non shmem backed objects

This patch adds support for extending the pread/pwrite functionality
for objects not backed by shmem. The access will be made through
gtt interface. This will cover objects backed by stolen memory as well
as other non-shmem backed objects.

v2: Drop locks around slow_user_access, prefault the pages before
access (Chris)

v3: Rebased to the latest drm-intel-nightly (Ankit)

v4: Moved page base & offset calculations outside the copy loop,
corrected data types for size and offset variables, corrected if-else
braces format (Tvrtko/kerneldocs)

v5: Enabled pread/pwrite for all non-shmem backed objects including
without tiling restrictions (Ankit)

v6: Using pwrite_fast for non-shmem backed objects as well (Chris)

v7: Updated commit message, Renamed i915_gem_gtt_read to i915_gem_gtt_copy,
added pwrite slow path for non-shmem backed objects (Chris/Tvrtko)

v8: Updated v7 commit message, mutex unlock around pwrite slow path for
non-shmem backed objects (Tvrtko)

v9: Corrected check during pread_ioctl, to avoid shmem_pread being
called for non-shmem backed objects (Tvrtko)

v10: Moved the write_domain check to needs_clflush and tiling mode check
to pwrite_fast (Chris)

v11: Use pwrite_fast fallback for all objects (shmem and non-shmem backed),
call fast_user_write regardless of pagefault in previous iteration

v12: Use page-by-page copy for slow user access too (Chris)

v13: Handled EFAULT, Avoid use of WARN_ON, put_fence only if whole obj
pinned (Chris)

v14: Corrected datatypes/initializations (Tvrtko)

Testcase: igt/gem_stolen, igt/gem_pread, igt/gem_pwrite

Signed-off-by: Ankitprasad Sharma <ankitprasad.r.sharma@intel.com>
Reviewed-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Signed-off-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1465548783-19712-1-git-send-email-ankitprasad.r.sharma@intel.com