anv: Set L3 full way allocation at context init if L3 cfg is NULL

anv: Set L3 full way allocation at context init if L3 cfg is NULL

If the platform's default L3 config is NULL, then it now gets
initialized only at context init time, and cmd_buffer_config_l3 will
always return immediately.

Rework:
 * Remove unneeded check on !cfg in cmd_buffer_config_l3 (Jason)

Signed-off-by: Jordan Justen <jordan.l.justen@intel.com>
Reviewed-by: Lionel Landwerlin <lionel.g.landwerlin@intel.com>
Reviewed-by: Jason Ekstrand <jason@jlekstrand.net>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/4956>