nvc0: serialize before updating some constant buffer bindings on Maxwell+

nvc0: serialize before updating some constant buffer bindings on Maxwell+

To avoid serializing, this has the user constant buffer always be 65536
bytes and enabled unless it's required that something else is used for
constant buffer 0.

Fixes artifacts with at least XCOM: Enemy Within, 0 A.D. and Unigine
Valley, Heaven and Superposition.

v2: changed uniform_buffer_bound to be bool instead of a uint32_t
v3: remove magic constants
v3: remove pointless code in nvc0_validate_driverconst

Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=100177
Signed-off-by: Rhys Perry <pendingchaos02@gmail.com>
Reviewed-by: Ilia Mirkin <imirkin@alum.mit.edu>