intel: Always downsample in intel_miptree_map_multisample

intel: Always downsample in intel_miptree_map_multisample

Always downsample before mapping, even if the map mode contains
GL_MAP_INVALIDATE_RANGE_BIT. If we neglect to downsample when only
a subrect is mapped then the upsample in intel_miptree_unmap_multisample
may write garbage to the region outside the subrect.

(Eric gave my patch e88cfbb a conditional reviewed-by with the condition
that it always downsample before mapping. I forgot to make that change
before pushing the patch.)

Signed-off-by: Chad Versace <chad.versace@linux.intel.com>