drm/i915: cleanup use of INSTR_CLIENT_MASK

Doing cmd_header >> 29 to extract our 3-bit client value where we know
cmd_header is a u32 shouldn't then also require the use of a mask. So
remove the redundant operation and get rid of INSTR_CLIENT_MASK now that
there are no longer any users.

Cc: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Matthew Auld <matthew.auld@intel.com>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1479163174-29686-1-git-send-email-matthew.auld@intel.com