drm/amdgpu: fix uninitialized variable pasid_mapping_needed

The boolean variable pasid_mapping_needed is not initialized and
there are code paths that do not assign it any value before it is
is read later.  Fix this by initializing pasid_mapping_needed to
false.

Addresses-Coverity: ("Uninitialized scalar variable")
Fixes: 6817bf28 ("drm/amdgpu: grab the id mgr lock while accessing passid_mapping")
Reviewed-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Colin Ian King <colin.king@canonical.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>