OMAPDSS: Add enum dss_pll_id

In some cases we need global identifiers for the DSS PLLs, for example
when configuring clock muxing on DRA7. For this purpose let's add a
'enum dss_pll_id'.

Signed-off-by: Tomi Valkeinen <tomi.valkeinen@ti.com>
diff --git a/drivers/video/fbdev/omap2/dss/dsi.c b/drivers/video/fbdev/omap2/dss/dsi.c
index 3e44c58..5081f6f 100644
--- a/drivers/video/fbdev/omap2/dss/dsi.c
+++ b/drivers/video/fbdev/omap2/dss/dsi.c
@@ -5238,6 +5238,7 @@
 	}
 
 	pll->name = dsi->module_id == 0 ? "dsi0" : "dsi1";
+	pll->id = dsi->module_id == 0 ? DSS_PLL_DSI1 : DSS_PLL_DSI2;
 	pll->clkin = clk;
 	pll->base = dsi->pll_base;