nv30: Disable msaa unless requested from the env by NV30_MAX_MSAA

nv30: Disable msaa unless requested from the env by NV30_MAX_MSAA

Some modern apps try to use msaa without keeping in mind the
restrictions on videomem of older cards. Resulting in dmesg saying:

 [ 1197.850642] nouveau E[soffice.bin[3785]] fail ttm_validate
 [ 1197.850648] nouveau E[soffice.bin[3785]] validating bo list
 [ 1197.850654] nouveau E[soffice.bin[3785]] validate: -12

Because we are running out of video memory, after which the program
using the msaa visual freezes, and eventually the entire system freezes.

To work around this we do not allow msaa visauls by default and allow
the user to override this via NV30_MAX_MSAA.

Signed-off-by: Hans de Goede <hdegoede@redhat.com>
[imirkin: move env var lookup to screen so that it's only done once]
Signed-off-by: Ilia Mirkin <imirkin@alum.mit.edu>
Cc: "10.6 11.0" <mesa-stable@lists.freedesktop.org>