mesa: Set gl_constants::MinMapBufferAlignment

mesa: Set gl_constants::MinMapBufferAlignment

Leaving it set to zero isn't really correct since every allocation has
at least an alignment of 1 byte.  It also caused a problem in the i965
driver after I removed the MAX(64, ...) from the alignment calculation.
That's what I get for changing a patch without retesting it. :(

Signed-off-by: Ian Romanick <ian.d.romanick@intel.com>
Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=73907
Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>
Cc: Lu Hua <huax.lu@intel.com>