drm/i915/bxt: Increase DDI buf idle timeout

For BXT, DDI buf idle timeout delay needs to be increased to 16us.

Since this is a timeout value and we return as soon as the condition is
realized, no penalty incurred for other platforms.

v2:
- remove TIMEOUT macro used only at a single place (Daniel)

Suggested-by: Satheeshakrishna M <satheeshakrishna.m@intel.com>
Cc: Satheeshakrishna M <satheeshakrishna.m@intel.com>
Cc: Damien Lespiau <damien.lespiau@intel.com>
Signed-off-by: Vandana Kannan <vandana.kannan@intel.com>
Signed-off-by: Damien Lespiau <damien.lespiau@intel.com> (v1)
Reviewed-by: Jani Nikula <jani.nikula@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>