drm/i915/selftests: Do not overwrite error code after intel_context_migrate_clear() call

Currently this function's error code is stored in err variable,
which, if a i915_request is present, will be immediately
overwritten by return from dma_resv_reserve_fences(). Call DMA
functions only if intel_context_migrate_clear() succeeded.

Suggested-by: Matthew Auld <matthew.auld@intel.com>
Signed-off-by: Krzysztof Karas <krzysztof.karas@intel.com>
Reviewed-by: Sebastian Brzezinka <sebastian.brzezinka@intel.com>
Reviewed-by: Andi Shyti <andi.shyti@linux.intel.com>
Signed-off-by: Andi Shyti <andi.shyti@linux.intel.com>
Link: https://lore.kernel.org/r/c0348e052bcb2870acdc15d60f5389fbdf4ef886.1753859971.git.krzysztof.karas@intel.com