intel/fs: Make alpha test work with MRT and sample mask

intel/fs: Make alpha test work with MRT and sample mask

Fix the order of src0_alpha and sample mask in fb payload.
From SKL PRM Volume 7, "Data Payload Register Order
for Render Target Write Messages":
 Type   S0A  oM  sZ  oS  M2     M3       M4
 SIMD8   1   1   0   0   s0A    oM       R
 SIMD16  1   1   0   0   1/0s0A 3/2s0A   oM

It also fixes working of alpha to coverage with sample mask
on GEN6 since now they are in correct order.

Signed-off-by: Danylo Piliaiev <danylo.piliaiev@globallogic.com>
Signed-off-by: Francisco Jerez <currojerez@riseup.net>
Reviewed-by:  Francisco Jerez <currojerez@riseup.net>