drm/amdgpu: Clean up of initializing doorbells for gfx_v9 and gfx_v10

Clean up redundant, copy-paste code blocks during the initialization of
the doorbells in mqd_init().

Signed-off-by: Haohui Mai <ricetons@gmail.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>