drm/i915: Check error return when setting DMA mask

Even though an error from these functions isn't fatal we still want to
have a diagnostic message about it.

v2:
- Don't do assignments in if statements. (Jani)

Cc: Jani Nikula <jani.nikula@intel.com>
Signed-off-by: Imre Deak <imre.deak@intel.com>
Reviewed-by: Jani Nikula <jani.nikula@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1494408113-379-4-git-send-email-imre.deak@intel.com