i965/fs: Set pixel/sample mask for compute shaders atomic ops

i965/fs: Set pixel/sample mask for compute shaders atomic ops

For fragment programs, we pull this mask from the payload header. The same
mask doesn't exist for compute shaders, so we set all bits to enabled.

Previously we were setting 0xff to support SIMD8 VS, but with CS we
support SIMD16, and therefore we change this to 0xffff.

Related commits for SIMD8 VS:

commit d9cd982d556be560af3bcbcdaf62b6b93eb934a5
Author: Ben Widawsky <benjamin.widawsky@intel.com>
Date:   Sun Feb 15 20:06:59 2015 -0800
    i965/simd8vs: Fix SIMD8 atomics

commit 4a95be9772a255776309f23180519a4a8560f2dd
Author: Jordan Justen <jordan.l.justen@intel.com>
Date:   Tue Feb 17 09:57:35 2015 -0800
    i965/simd8vs: Fix SIMD8 atomics (read-only)

Note: this mask is ANDed with the execution mask, so some channels may not end
up issuing the atomic operation.

Signed-off-by: Jordan Justen <jordan.l.justen@intel.com>
Reviewed-by: Ben Widawsky <ben@bwidawsk.net>
Reviewed-by: Francisco Jerez <currojerez@riseup.net>