drm/i915: Remove redundant const from function return type

Marking function return types as const is redundant, as these are
rvalues and as such constant by definition. Code checkers and GCC will
warn about this so remove the modifier.

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/1463059132-1720-5-git-send-email-imre.deak@intel.com