drm/amdgpu: fix coding style and printing in amdgpu_doorbell_init

Based on commit "drm/radeon: remove useless and potentially wrong message".

The size of the info printing is incorrect and the PCI subsystems prints
the same info on boot anyway.

Signed-off-by: Christian König <christian.koenig@amd.com>
Reviewed-by: Edward O'Callaghan <funfunctor@folklore1984.net>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>