drm/amdgpu: read sdma edc counter to clear the counters

SDMA edc counter registers were added in gfx edc counters
array. When querying gfx error counter in that array, there
is no way to differentiate sdma instance number for different
asic and then results to NULL pointer access when trying to
read sdma register base address for instances greater
than 2 on Vega20.
In addition, this also results to wrong gfx error counters
since it actually added sdma edc counters.
Therefore, sdma edc counter registers should be separated
from gfx edc counter regsiter array and only get initialized
when driver tries to enable sdma ras.

Signed-off-by: Hawking Zhang <Hawking.Zhang@amd.com>
Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>