drm/i915: Merge of visible and !visible paths for primary planes

Fold intel_pipe_set_base() in the update primary plane path merging
pieces of code that are common to both paths.

Basically the the pin/unpin procedures are the same for both paths
and some checks can also be shared (some of the were moved to the
check() stage)

v2: take Ville's comments:
	- remove unnecessary plane check
	- move mutex lock to inside the conditional
	- make the pin fail message a debug one
	- add a fixme for the fastboot hack
	- call intel_frontbuffer_flip() after FBC update

v3: take more Ville's comments:
	- fold update code under if (intel_crtc->active), and do the
	visible/!visible split inside.
	- check ret inside the same conditional we assign it

v4: don't use intel_enable_primary_hw_plane(), the primary_enabled
check inside will break page flips

v5: take more Ville's comments:
	- set primary_enabled to true and add BDW hack
	- unify if (old_fb) and if (old_fb != fb)

v6: take more Ville's comments:
	- make was_primary bool and fix its check
	- add the BDW vblank wait comment

Suggested-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Gustavo Padovan <gustavo.padovan@collabora.co.uk>
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>