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: a6e7a006 ("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.comReviewed-by: Manasi Navare <manasi.d.navare@intel.com>

drivers/gpu/drm/i915/display/intel_display.c

@@ -346,7 +346,10 @@ static enum pipe bigjoiner_master_pipe(const struct intel_crtc_state *crtc_state
 
 u8 intel_crtc_bigjoiner_slave_pipes(const struct intel_crtc_state *crtc_state)
 {
-	return crtc_state->bigjoiner_pipes & ~BIT(bigjoiner_master_pipe(crtc_state));
+	if (crtc_state->bigjoiner_pipes)
+		return crtc_state->bigjoiner_pipes & ~BIT(bigjoiner_master_pipe(crtc_state));
+	else
+		return 0;
 }
 
 bool intel_crtc_is_bigjoiner_slave(const struct intel_crtc_state *crtc_state)