drm/amd/display: Fix memory leak in dm_sw_fini()

After destroying dmub_srv, the memory associated with it is
not freed, causing a memory leak:

unreferenced object 0xffff896302b45800 (size 1024):
  comm "(udev-worker)", pid 222, jiffies 4294894636
  hex dump (first 32 bytes):
    00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00  ................
    00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00  ................
  backtrace (crc 6265fd77):
    [<ffffffff993495ed>] kmalloc_trace+0x29d/0x340
    [<ffffffffc0ea4a94>] dm_dmub_sw_init+0xb4/0x450 [amdgpu]
    [<ffffffffc0ea4e55>] dm_sw_init+0x15/0x2b0 [amdgpu]
    [<ffffffffc0ba8557>] amdgpu_device_init+0x1417/0x24e0 [amdgpu]
    [<ffffffffc0bab285>] amdgpu_driver_load_kms+0x15/0x190 [amdgpu]
    [<ffffffffc0ba09c7>] amdgpu_pci_probe+0x187/0x4e0 [amdgpu]
    [<ffffffff9968fd1e>] local_pci_probe+0x3e/0x90
    [<ffffffff996918a3>] pci_device_probe+0xc3/0x230
    [<ffffffff99805872>] really_probe+0xe2/0x480
    [<ffffffff99805c98>] __driver_probe_device+0x78/0x160
    [<ffffffff99805daf>] driver_probe_device+0x1f/0x90
    [<ffffffff9980601e>] __driver_attach+0xce/0x1c0
    [<ffffffff99803170>] bus_for_each_dev+0x70/0xc0
    [<ffffffff99804822>] bus_add_driver+0x112/0x210
    [<ffffffff99807245>] driver_register+0x55/0x100
    [<ffffffff990012d1>] do_one_initcall+0x41/0x300

Fix this by freeing dmub_srv after destroying it.

Fixes: 743b9786 ("drm/amd/display: Hook up the DMUB service in DM")
Signed-off-by: Armin Wolf <W_Armin@gmx.de>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>