drm/i915: Rename similar plane functions to avoid confusion

The name 'update_plane' was used both for the primary plane functions in
intel_display.c and the sprite/overlay functions in intel_sprite.c.
Rename the primary plane functions to 'update_primary_plane' to avoid
confusion.

On a similar note, intel_display.c already had a function called
intel_disable_primary_plane() that programs the hardware to disable a
pipe's primary plane.  When we hook up primary planes through the DRM
plane interface, one of the natural handler names will be
intel_primary_plane_disable(), which is very similar.  To avoid
confusion, rename the existing intel_disable_primary_plane() to
intel_disable_primary_hw_plane() to make the two names a little more
distinct.

Cc: Intel Graphics Development <intel-gfx@lists.freedesktop.org>
Signed-off-by: Matt Roper <matthew.d.roper@intel.com>
[danvet: Fix up conflicts.]
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>