anv/pipeline/gen8: Unconditionally set DXMultisampleRasterizaitonEnable

anv/pipeline/gen8: Unconditionally set DXMultisampleRasterizaitonEnable

The multisample rasterization mode is computed based on this field,
3DSTATE_RASTER::DXMultisampleRasterizationMode (only for forced
multisampling), 3DSTATE_RASTER::APIMode, and the number of samples.  There
are two tables in the SKL PRM that describe how the final multisample mode
is calculated: "Windower (WM) Stage >> Multisampling >> Multisample
ModeState >> Table 1" and the formula for "SF_INT::Multisample
Rasterization Mode".

The "DX Multisample Rasterization Enable" bit changes whether multisample
mode is set to OFF_PIXEL or ON_PATTERN in the samples > 1 case.  In the
samples == 1 case, the bit has no effect.  Since Vulkan has no concept of
disabling multisampling for samples > 1, we can just set the bit.

Signed-off-by: Jason Ekstrand <jason@jlekstrand.net>
Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>