drm/amdkfd: fix NULL ptr for debugfs mqds on GFX v12

mqd_stride function in gfx v12 is not implemented, that
causes NULL ptr error. Add the generic func to fix it.

Signed-off-by: Eric Huang <jinhuieric.huang@amd.com>
Reviewed-by: Harish Kasiviswanathan <Harish.Kasiviswanathan@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>