drm/amdgpu: adjust default dispclk (v2)

Set the default to 600Mhz if it's not set in the bios,
and bump the default to 600Mhz if it's lower than that.

Port of radeon commit:
9368931db826d57b6b88b3145a00276626b48df0

v2: clean up the code a bit

bug:
https://bugs.freedesktop.org/show_bug.cgi?id=91896

Signed-off-by: Alex Deucher <[e-mail]>

drivers/gpu/drm/amd/amdgpu/amdgpu_atombios.c

···
 		/* disp clock */
 		adev->clock.default_dispclk =
 			le32_to_cpu(firmware_info->info_21.ulDefaultDispEngineClkFreq);
-		if (adev->clock.default_dispclk == 0)
-			adev->clock.default_dispclk = 54000; /* 540 Mhz */
+		/* set a reasonable default for DP */
+		if (adev->clock.default_dispclk < 53900) {
+			DRM_INFO("Changing default dispclk from %dMhz to 600Mhz\n",
+				 adev->clock.default_dispclk / 100);
+			adev->clock.default_dispclk = 60000;
+		}
 		adev->clock.dp_extclk =
 			le16_to_cpu(firmware_info->info_21.usUniphyDPModeExtClkFreq);
 		adev->clock.current_dispclk = adev->clock.default_dispclk;