drm/amd: Use `amdgpu_ucode_*` helpers for GFX6

The `amdgpu_ucode_request` helper will ensure that the return code for
missing firmware is -ENODEV so that early_init can fail.

The `amdgpu_ucode_release` helper is for symmetry on unloading.

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