drm/amd/amdgpu: Remove initialisation of globals to 0 or NULL

Global variables do not need to be initialized to 0 or NULL and checkpatch
flags this error in drivers/gpu/drm/amd/amdgpu/amdgpu_drv.c:

ERROR: do not initialise globals to NULL
+char *amdgpu_disable_cu = NULL;
+char *amdgpu_virtual_display = NULL;

Fix this checkpatch error.

Cc: Christian König <christian.koenig@amd.com>
Cc: Alex Deucher <alexander.deucher@amd.com>
Cc: Mario Limonciello <mario.limonciello@amd.com>
Cc: Hamza Mahfooz <Hamza.Mahfooz@amd.com>
Signed-off-by: Srinivasan Shanmugam <srinivasan.shanmugam@amd.com>
Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>