drm/edid: pass a timing pointer to is_detailed_timing_descriptor()

Use struct member access instead of direct offsets to avoid a cast.

Use BUILD_BUG_ON() for sanity check.

Cc: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Reviewed-by: Emil Velikov <emil.l.velikov@gmail.com>
Link: https://patchwork.freedesktop.org/patch/msgid/0b5213383e14f11c6a505b10a7342fb2ff4f2a11.1648477901.git.jani.nikula@intel.com