amdgpu: Add new function to get fd.

Dual purpose:
 - The drm fd dedupe functionality confuses the radeonsi
   amdgpu winsys if radeonsi isn't the first thing opening
   the device. By exposing the fd we can detect this case.
 - For a common mesa Vulkan sync objects implementation
   with syncobj. (notable: no buffer allocation)

Both shouldn't interferece with libdrm_amdgpu functionality
though it does somewhat piece the abstraction of the library.

Signed-off-by: Bas Nieuwenhuizen <bas@basnieuwenhuizen.nl>
Gitlab: https://gitlab.freedesktop.org/mesa/mesa/-/issues/3424
Gitlab: https://gitlab.freedesktop.org/mesa/mesa/-/issues/5630
Reviewed-by: Marek Olšák <marek.olsak@amd.com>

amdgpu/amdgpu-symbols.txt

@@ -53,6 +53,7 @@ amdgpu_cs_syncobj_wait
 amdgpu_cs_wait_fences
 amdgpu_cs_wait_semaphore
 amdgpu_device_deinitialize
+amdgpu_device_get_fd
 amdgpu_device_initialize
 amdgpu_find_bo_by_cpu_mapping
 amdgpu_get_marketing_name

amdgpu/amdgpu.h

@@ -546,6 +546,19 @@ int amdgpu_device_initialize(int fd,
 */
 int amdgpu_device_deinitialize(amdgpu_device_handle device_handle);
 
+/**
+ *
+ * /param device_handle - \c [in] Device handle.
+ *                           See #amdgpu_device_initialize()
+ *
+ * \return Returns the drm fd used for operations on this
+ *         device. This is still owned by the library and hence
+ *         should not be closed. Guaranteed to be valid until
+ *         #amdgpu_device_deinitialize gets called.
+ *
+*/
+int amdgpu_device_get_fd(amdgpu_device_handle device_handle);
+
 /*
  * Memory Management
  *

amdgpu/amdgpu_device.c

@@ -285,6 +285,11 @@ drm_public int amdgpu_device_deinitialize(amdgpu_device_handle dev)
 	return 0;
 }
 
+drm_public int amdgpu_device_get_fd(amdgpu_device_handle device_handle)
+{
+	return device_handle->fd;
+}
+
 drm_public const char *amdgpu_get_marketing_name(amdgpu_device_handle dev)
 {
 	return dev->marketing_name;