drm: Remove compiler BUG_ON() test

modeset->num_connectors must be 0 to reach the BUG_ON() which tests
for non-zero modeset->num_connectors; remove BUG_ON().

Signed-off-by: Peter Hurley <peter@hurleysoftware.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>