drm/amdgpu: Fix size calculation when init onchip memory

Size is page count here.

Closes: https://gitlab.freedesktop.org/drm/amd/-/issues/1372
Reviewed-by: Christian König <christian.koenig@amd.com>
Signed-off-by: xinhui pan <xinhui.pan@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>
(cherry picked from commit d836917d

)
[airlied: from drm-next]
Signed-off-by: Dave Airlie <airlied@redhat.com>