drm/i915: Set the primary plane pipe select bits on gen4

drm/i915: Set the primary plane pipe select bits on gen4

i965 and g4x still have the pipe select bits in the plane control
registers, they're just hardcoded to select a specific pipe. However
plane C on i965 can still move between the pipes, thus we should
program the pipe select bits on i965 if we want to expose plane C
some day.

Since there is no harm in programming the bits on any plane on
i965/g4x let's just always set them. This will also make our
pre-computed register value match what the hardware register
would read, should we want to cross check the two.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20180130203807.13721-2-ville.syrjala@linux.intel.com
Reviewed-by: Mika Kahola <mika.kahola@intel.com>