drm/i915: Consolidate plane 'prepare' functions (v2)

The 'prepare' step for all types of planes are pretty similar;
consolidate the three 'prepare' functions into a single function.  This
paves the way for future integration with the atomic plane handlers.

Note that we pull the 'wait for pending flips' functionality out of the
primary plane's prepare step and place it directly in the 'setplane'
code.  When we move to the atomic plane handlers, this code will be in
the 'atomic begin' step.

v2: Update GEM fb tracking for physical cursors also (Ander)

Signed-off-by: Matt Roper <matthew.d.roper@intel.com>
Reviewed-by: Ander Conselvan de Oliveira <ander.conselvan.de.oliveira@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>