drm/i915: Protect PIPE_CONF_CHECK macros with do {} while(0)

Make the PIPE_CONF_CHECK macros a bit more robust by wrapping them
in do {} while(0). Avoids funky sirprises when you try put an 'else'
after a PIPE_CONF_CHECK invocation...

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20180316183625.16316-1-ville.syrjala@linux.intel.com
Reviewed-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com> #irc