i965: take the target into account for Gen7 MSAA modes

i965: take the target into account for Gen7 MSAA modes

Gen7 has an erratum affecting the ld_mcs message, making it unsafe to
use when the surface doesn't have an associated MCS.

From the Ivy Bridge PRM, Vol4 Part1 p77 ("MCS Enable"):

   "If this field is disabled and the sampling engine <ld_mcs>
   message is issued on this surface, the MCS surface may be
   accessed. Software must ensure that the surface is defined
   to avoid GTT errors."

To allow the shader to treat all surfaces uniformly, force UMS if the
surface is to be used as a multisample texture, even if CMS would have
been possible.

V3: - Quoted erratum text

Signed-off-by: Chris Forbes <chrisf@ijw.co.nz>
Reviewed-by: Eric Anholt <eric@anholt.net>