drm/i915: Add PIPE_CONF_CHECK_CLOCK_FUZZY()

Add a new pipe config check macro PIPE_CONF_CHECK_CLOCK_FUZZY() to make
it trivial and error proof to compare clocks in a fuzzy manner.

v2: Drop extra curly braces

Reviewed-by: Jani Nikula <jani.nikula@intel.com>
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>