drm/amd: Drop error message about failing to load DMUB firmware

The error path for DMUB firmware loading is unnecessarily noisy.
When a firmware is missing 3 errors show up:
```
amdgpu 0000:07:00.0: Direct firmware load for amdgpu/green_sardine_dmcub.bin failed with error -2
[drm:dm_early_init [amdgpu]] *ERROR* DMUB firmware loading failed: -19
[drm:amdgpu_device_init [amdgpu]] *ERROR* early_init of IP block <dm> failed -19
```

The error code for the device init is bubbled up already, remove the
second one.

Signed-off-by: Mario Limonciello <mario.limonciello@amd.com>
Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>