drm: Help unconfuse gcc, avoid accidental impossible unsigned comparisons

drivers/gpu/drm/drm_client_modeset.c: In function ‘drm_client_firmware_config’:
./include/linux/bits.h:26:28: warning: comparison of unsigned expression < 0 is always false [-Wtype-limits]
   __builtin_constant_p((l) > (h)), (l) > (h), 0)))

v2: Add a warning for passing connector_count==0 as this will hit an
infinite loop, so document the invalid parameter.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: https://patchwork.freedesktop.org/patch/msgid/20200516212330.13633-2-chris@chris-wilson.co.uk