drm/amdgpu/uvd6: De-numberify startup

To make the code more legible various numerical constants
have been changed to their #define'ed MASKs.

Signed-off-by: Tom St Denis <tom.stdenis@amd.com>
Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>