drm/i915: Look at staged config when fixing pipe_src_w for LVDS

The code in function intel_crtc_compute_config() that evens pipe_src_w
if necessary would look at the current config instead of the staged one
when deciding if there is an LVDS encoder in use. This could potentially
lead to the value not being updated, if during the modeset a crtc wasn't
driving an LVDS encoder.

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