drm/radeon/kms: fix divide by 0 in clocks code

If the chip isn't initialised properly this can happen.
also fix return value in combios clocks function.
Signed-off-by: Dave Airlie <airlied@redhat.com>

drivers/gpu/drm/radeon/radeon_clocks.c

@@ -44,6 +44,10 @@ uint32_t radeon_legacy_get_engine_clock(struct radeon_device *rdev)
 
 	ref_div =
 	    RREG32_PLL(RADEON_M_SPLL_REF_FB_DIV) & RADEON_M_SPLL_REF_DIV_MASK;
+
+	if (ref_div == 0)
+		return 0;
+
 	sclk = fb_div / ref_div;
 
 	post_div = RREG32_PLL(RADEON_SCLK_CNTL) & RADEON_SCLK_SRC_SEL_MASK;
@@ -70,6 +74,10 @@ static uint32_t radeon_legacy_get_memory_clock(struct radeon_device *rdev)
 
 	ref_div =
 	    RREG32_PLL(RADEON_M_SPLL_REF_FB_DIV) & RADEON_M_SPLL_REF_DIV_MASK;
+
+	if (ref_div == 0)
+		return 0;
+
 	mclk = fb_div / ref_div;
 
 	post_div = RREG32_PLL(RADEON_MCLK_CNTL) & 0x7;

drivers/gpu/drm/radeon/radeon_combios.c

@@ -495,7 +495,7 @@ bool radeon_combios_get_clock_info(struct drm_device *dev)
 	uint16_t sclk, mclk;
 
 	if (rdev->bios == NULL)
-		return NULL;
+		return false;
 
 	pll_info = combios_get_table_offset(dev, COMBIOS_PLL_INFO_TABLE);
 	if (pll_info) {