drm/amd: Add the capability to mark certain firmware as "required"

Some of the firmware that is loaded by amdgpu is not actually required.
For example the ISP firmware on some SoCs is optional, and if it's not
present the ISP IP block just won't be initialized.

The firmware loader core however will show a warning when this happens
like this:
```
Direct firmware load for amdgpu/isp_4_1_0.bin failed with error -2
```

To avoid confusion for non-required firmware, adjust the amd-ucode helper
to take an extra argument indicating if the firmware is required or
optional.

On optional firmware use firmware_request_nowarn() instead of
request_firmware() to avoid the warnings.

Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Link: https://lore.kernel.org/amd-gfx/df71d375-7abd-4b32-97ce-15e57846eed8@amd.com/T/#t


Signed-off-by: Mario Limonciello <mario.limonciello@amd.com>