drm/amdkfd: Initialize SDMA activity counter to 0

To prevent reporting erroneous SDMA usage, initialize SDMA
activity counter to 0 before using.

Signed-off-by: Mukul Joshi <mukul.joshi@amd.com>
Reviewed-by: Felix Kuehling <Felix.Kuehling@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>