drm/i915: Make use of intel_fb_obj() (v2)

This should hopefully simplify the display code slightly and also
solves at least one mistake in intel_pipe_set_base() where
to_intel_framebuffer(fb)->obj is referenced during local variable
initialization, before 'if (!fb)' gets checked.

Potential uses of this macro were identified via the following
Coccinelle patch:

        @@
        expression E;
        @@
        * to_intel_framebuffer(E)->obj

        @@
        expression E;
        identifier I;
        @@
          I = to_intel_framebuffer(E);
          ...
        * I->obj

v2: Rewrite some NULL tests in terms of the obj rather than the fb.
    Also add a WARN() if trying to pageflip with a disabled primary
    plane.  [Suggested by Chris Wilson]

Signed-off-by: Matt Roper <matthew.d.roper@intel.com>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>