i965/miptree: Directly gtt map the mcs buffer

i965/miptree: Directly gtt map the mcs buffer

The next patch will change the map type, and this will make sure there are no
regressions as a result of the other stuff. Since the miptree is newly created,
I believe it is always safe to just map.

It is possible to CPU map this buffer on LLC platforms (it additionally requires
rounding up to tile size). I did experiment with that patch, and found no
performance gains to be had.

I've added in error handling while here. Generally GTT mapping is an operation
which is highly unlikely to fail, but we may as well handle it when it does.

v2: rebase on master (Lionel)

v3: print out error if gtt mapping fails (Topi)

Signed-off-by: Ben Widawsky <benjamin.widawsky@intel.com> (v1)
Signed-off-by: Lionel Landwerlin <lionel.g.landwerlin@intel.com> (v2)
Reviewed-by: Topi Pohjolainen <topi.pohjolainen@intel.com>