drm/gma500: remove set but not used variables 'hist_reg'

Fixes gcc '-Wunused-but-set-variable' warning:

drivers/gpu/drm/gma500/psb_irq.c: In function psb_irq_turn_off_dpst:
drivers/gpu/drm/gma500/psb_irq.c:473:6:
	warning: variable hist_reg set but not used [-Wunused-but-set-variable]
Reported-by: Hulk Robot <hulkci@huawei.com>
Signed-off-by: Chen Zhou <chenzhou10@huawei.com>
Signed-off-by: Patrik Jakobsson <patrik.r.jakobsson@gmail.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20191227114811.14907-1-chenzhou10@huawei.com

drivers/gpu/drm/gma500/psb_irq.c

@@ -470,12 +470,11 @@ void psb_irq_turn_off_dpst(struct drm_device *dev)
 {
 	struct drm_psb_private *dev_priv =
 	    (struct drm_psb_private *) dev->dev_private;
-	u32 hist_reg;
 	u32 pwm_reg;
 
 	if (gma_power_begin(dev, false)) {
 		PSB_WVDC32(0x00000000, HISTOGRAM_INT_CONTROL);
-		hist_reg = PSB_RVDC32(HISTOGRAM_INT_CONTROL);
+		PSB_RVDC32(HISTOGRAM_INT_CONTROL);
 
 		psb_disable_pipestat(dev_priv, 0, PIPE_DPST_EVENT_ENABLE);