drm/i915: Compute DSI PLL parameters during .compute_config()

Compute the DSI PLL parameters during .compute_config() rather than
.pre_pll_enable() so that we can fail gracefully if we can't find
suitable parameters.

In order to do that we need to store the DSI PLL parameters in
pipe_config.

v2: Handle BXT too

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Reviewed-by: Jani Nikula <jani.nikula@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1460488478-18311-3-git-send-email-ville.syrjala@linux.intel.com
Tested-by: Jani Nikula <jani.nikula@intel.com>
diff --git a/drivers/gpu/drm/i915/intel_drv.h b/drivers/gpu/drm/i915/intel_drv.h
index 7215335..e13ce22 100644
--- a/drivers/gpu/drm/i915/intel_drv.h
+++ b/drivers/gpu/drm/i915/intel_drv.h
@@ -497,6 +497,11 @@
 	/* Actual register state of the dpll, for shared dpll cross-checking. */
 	struct intel_dpll_hw_state dpll_hw_state;
 
+	/* DSI PLL registers */
+	struct {
+		u32 ctrl, div;
+	} dsi_pll;
+
 	int pipe_bpp;
 	struct intel_link_m_n dp_m_n;