drm/i915: Fix all intel_framebuffer_init failures to take the error path

No more direct return -EINVAL as we have to unwind the
obj->framebuffer_references.

Fixes: 24dbf51a ("drm/i915: struct_mutex is not required for allocating the framebuffer")
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Ville Syrjälä <ville.syrjala@linux.intel.com>
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/20170301154128.2841-1-chris@chris-wilson.co.uk