i965/blorp: Configure SURFACE_STATE correctly for IMS surfaces.

This patch modifies gen7_set_surface_num_multisamples() to set up the
SURFACE_STATE appropriately for texturing from IMS format MSAA
surfaces (which are only used on Gen7 for depth and stencil buffers).
Since the function now sets more than just the number of multisamples,
it's been renamed to gen7_set_surface_msaa().

This will make it possible to remove some kludginess from the blorp
engine.

Reviewed-by: Anuj Phogat <anuj.phogat@gmail.com>