drm/i915: Always load the display palette before enabling the pipe

Loading the palette after the planes are enabled can risk showing
incorrect colors. ILK+ already load the palette before even the pipe
is enabled. Just follow the same order for gen2-4 and VLV.

According to BSpec the requirements for palette access are
display core clock and display PLL running. In certain platforms
just the core clock may be enough. But we definitely should have both
running when this gets called during the modeset.

v2: Amend the commit message with some display PLL/core clock info

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Reviewed-by: Rodrigo Vivi <rodrigo.vivi@gmail.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>