drm/i915/cnl: Mask previous DDI - PLL mapping

Without masking out the old value, we can end up pointing the DDI to a
disabled PLL, which makes the system fall over. Mask out the previous
value before setting the PLL to DDI mapping.

This can be observed by running igt/testdisplay with both an eDP and
HDMI/DP output active.

v2: Add the Bugzilla link

Fixes: 555e38d2 ("drm/i915/cnl: DDI - PLL mapping")
Testcase: igt/testdisplay
Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=103997
Cc: Rodrigo Vivi <rodrigo.vivi@intel.com>
Cc: Matt Atwood <matthew.s.atwood@intel.com>
Signed-off-by: James Ausmus <james.ausmus@intel.com>
Reviewed-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Tested-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Signed-off-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20171201021700.13504-1-james.ausmus@intel.com

drivers/gpu/drm/i915/intel_ddi.c

@@ -2098,6 +2098,7 @@ static void intel_ddi_clk_select(struct intel_encoder *encoder,
 	if (IS_CANNONLAKE(dev_priv)) {
 		/* Configure DPCLKA_CFGCR0 to map the DPLL to the DDI. */
 		val = I915_READ(DPCLKA_CFGCR0);
+		val &= ~DPCLKA_CFGCR0_DDI_CLK_SEL_MASK(port);
 		val |= DPCLKA_CFGCR0_DDI_CLK_SEL(pll->id, port);
 		I915_WRITE(DPCLKA_CFGCR0, val);