drm/i915: Handle error return from dma_set_coherent_mask()

A failure from these functions can lead to obscure bugs later, so it's
better not to suppress them and just fail module loading.

Signed-off-by: Imre Deak <imre.deak@intel.com>
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1463059132-1720-4-git-send-email-imre.deak@intel.com