drm/i915: Avoid negative shift due to bigjoiner_pipes==0

drm/i915: Avoid negative shift due to bigjoiner_pipes==0

bigjoiner_pipes==0 leads bigjoiner_master_pipe() to
do BIT(ffs(0)-1) which is undefined behaviour. The code should
actually still work fine since the only place we provoke
that is intel_crtc_bigjoiner_slave_pipes() and it'll bitwise
AND the result with 0, so doesn't really matter what we get
out of bigjoiner_master_pipe(). But best not provoke undefined
behaviour anyway.

Reported-by: kernel test robot <oliver.sang@intel.com>
Fixes: a6e7a006f5d5 ("drm/i915: Change bigjoiner state tracking to use the pipe bitmask")
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20220223131315.18016-2-ville.syrjala@linux.intel.com
Reviewed-by: Manasi Navare <manasi.d.navare@intel.com>