drm/amdgpu/sdma: Fix incorrect calculations of the wptr of the doorbells

This patch fixes the issue where the driver miscomputes the 64-bit
values of the wptr of the SDMA doorbell when initializing the
hardware. SDMA engines v4 and later on have full 64-bit registers for
wptr thus they should be set properly.

Older generation hardwares like CIK / SI have only 16 / 20 / 24bits
for the WPTR, where the calls of lower_32_bits() will be removed in a
following patch.

Reviewed-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Haohui Mai <ricetons@gmail.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>