drm/i915: relax uncritical udelay_range()

udelay_range(1, 2) is inefficient and as discussions with Jani Nikula
<jani.nikula@linux.intel.com> unnecessary here. This replaces this
tight setting with a relaxed delay of min=20 and max=50 which helps
the hrtimer subsystem optimize timer handling.

Fixes: commit be4fc046bed3 ("drm/i915: add VLV DSI PLL Calculations")
Link: http://lkml.org/lkml/2016/12/15/147
Signed-off-by: Nicholas Mc Guire <hofrat@osadl.org>
Reviewed-by: Jani Nikula <jani.nikula@intel.com>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1481853578-19834-1-git-send-email-hofrat@osadl.org

drivers/gpu/drm/i915/intel_dsi_pll.c

@@ -156,8 +156,10 @@ static void vlv_enable_dsi_pll(struct intel_encoder *encoder,
 	vlv_cck_write(dev_priv, CCK_REG_DSI_PLL_CONTROL,
 		      config->dsi_pll.ctrl & ~DSI_PLL_VCO_EN);
 
-	/* wait at least 0.5 us after ungating before enabling VCO */
-	usleep_range(1, 10);
+	/* wait at least 0.5 us after ungating before enabling VCO,
+	 * allow hrtimer subsystem optimization by relaxing timing
+	 */
+	usleep_range(10, 50);
 
 	vlv_cck_write(dev_priv, CCK_REG_DSI_PLL_CONTROL, config->dsi_pll.ctrl);