drm/i915: Error out if we are trying to use VGA with SPLL already in use

Our static analysis tool noticed that 'reg' could be used uninitialized if
we are trying to get a PLL to drive VGA and SPLL is already in use
(plls->spll_refcoung != 0).

In the (error) case above, let's return false to the caller and emit an
error.
Signed-off-by: Damien Lespiau <damien.lespiau@intel.com>
Reviewed-by: Paulo Zanoni <paulo.r.zanoni@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>

drivers/gpu/drm/i915/intel_ddi.c

@@ -898,6 +898,9 @@ bool intel_ddi_pll_mode_set(struct drm_crtc *crtc, int clock)
 			plls->spll_refcount++;
 			reg = SPLL_CTL;
 			intel_crtc->ddi_pll_sel = PORT_CLK_SEL_SPLL;
+		} else {
+			DRM_ERROR("SPLL already in use\n");
+			return false;
 		}
 
 		WARN(I915_READ(reg) & SPLL_PLL_ENABLE,