drm/i915: Clean up skl_program_scaler()

Remove the "sizes are 0 based" stuff that is not even true for the
scaler.

v2: Rebase

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20181101151736.20522-1-ville.syrjala@linux.intel.com
Reviewed-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
(cherry picked from commit d0105af9

)
Signed-off-by: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>