drm/radeon: add a module parameter to control deep color support

Some monitors seem to have problems with deep color enabled, even
though they claim to support it.  I'm not sure if the monitor
need a quirk or if the driver is doing something the monitor doesn't
like.  At this point lets just disable deep color by default like
we did for hdmi audio and work through the bugs so we can eventually
enable it by default.

bug:
https://bugs.freedesktop.org/show_bug.cgi?id=80531

Signed-off-by: Alex Deucher <alexander.deucher@amd.com>
diff --git a/drivers/gpu/drm/radeon/radeon_drv.c b/drivers/gpu/drm/radeon/radeon_drv.c
index 6e30174..cb14213 100644
--- a/drivers/gpu/drm/radeon/radeon_drv.c
+++ b/drivers/gpu/drm/radeon/radeon_drv.c
@@ -175,6 +175,7 @@
 int radeon_hard_reset = 0;
 int radeon_vm_size = 4096;
 int radeon_vm_block_size = 9;
+int radeon_deep_color = 0;
 
 MODULE_PARM_DESC(no_wb, "Disable AGP writeback for scratch registers");
 module_param_named(no_wb, radeon_no_wb, int, 0444);
@@ -248,6 +249,9 @@
 MODULE_PARM_DESC(vm_block_size, "VM page table size in bits (default 9)");
 module_param_named(vm_block_size, radeon_vm_block_size, int, 0444);
 
+MODULE_PARM_DESC(deep_color, "Deep Color support (1 = enable, 0 = disable (default))");
+module_param_named(deep_color, radeon_deep_color, int, 0444);
+
 static struct pci_device_id pciidlist[] = {
 	radeon_PCI_IDS
 };