drm/amdgpu: do not initialise global variables to 0 or NULL

Initializing global variable to 0 or NULL is not necessary and should
be avoided. Issue reported by checkpatch script as:
ERROR: do not initialise globals to 0 (or NULL).

Reviewed-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Deepak R Varma <mh12gx2825@gmail.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>