drm: Add drm_mode_equal_no_clocks()

drm_mode_equal_no_clocks() is like drm_mode_equal() except it doesn't
compare the clock or vrefresh values. drm_mode_equal() is now
implemented by first doing the clock checks, and then calling
drm_mode_equal_no_clocks().

v2: Add missing EXPORT_SYMBOL()

Reviewed-by: Paulo Zanoni <paulo.r.zanoni@intel.com>
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>