drm/i915: Move DIV_ROUND_CLOSEST_ULL macro to header

Move the duplicated DIV_ROUND_CLOSEST_ULL macro into the intel_drv.h
header file so that it can be shared between intel_display.c
and intel_panel.c.

Signed-off-by: U. Artie Eoff <ullysses.a.eoff@intel.com>
Reviewed-By: Joe Konno <joe.konno@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>