drm/amdgpu: assign the doorbell index in 1st page to sdma page queue

Previously for vega10, the sdma_doorbell_range is only enough for sdma
gfx queue, thus the index on second doorbell page is allocated for sdma
page queue. From vega20, the sdma_doorbell_range on 1st page is enlarged.
Therefore, just leverage these index instead of allocation on 2nd page.

v2: change "(x << 1) + 2" to "(x + 1) << 1" for readability and add comments.

Signed-off-by: Le Ma <le.ma@amd.com>
Acked-by: Felix Kuehling <Felix.Kuehling@amd.com>
Reviewed-by: Lijo Lazar <lijo.lazar@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>