drm/i915: simplify setting DSPCNTR inside ironlake_crtc_mode_set

Because declaring a variable in the beginning of the function, then
initializing it 100 lines later, then using it 100 lines later does
not make our code look good IMHO.

Signed-off-by: Paulo Zanoni <paulo.r.zanoni@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>