drm/i915: Move fb pinning into __intel_set_mode

Our two ->crtc_mode_set callbacks really don't care whether the fb is
pinned and set up already or not - all the state computation and
handling which originally looked at the framebuffer is already using
the indirection through the pipe configuration.

Eventually we want to move this up a bit more, but as long as the crtc
mode_set callback still exists (and as long as we don't need to pin an
entire pile of planes due to atomic modesets) there's not much point
in it. So I'll let this be for now.

v2: Don't forget about haswell ...

Reviewed-by: Akash Goel <akash.goel@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>