iris: initialize shared screen->vtbl only once

iris: initialize shared screen->vtbl only once

Screen is shared among contexts, other context might be already using
vtbl while another initializes it again.

 ==45872== Possible data race during write of size 8 at 0x5DDAE78 by thread #549
 ==45872== Locks held: 1, at address 0x5D1B6F8
 ==45872==    at 0x6D66D91: gen9_init_state (iris_state.c:7816)
 ==45872==    by 0x6BA0A31: iris_create_context (iris_context.c:342)
 ==45872==    by 0x621F390: st_api_create_context (st_manager.c:917)
 ==45872==    by 0x620E6F9: dri_create_context (dri_context.c:163)
 ==45872==    by 0x6A40DB1: driCreateContextAttribs (dri_util.c:480)
 ==45872==    by 0x540B963: dri2_create_context (egl_dri2.c:1583)
 ==45872==    by 0x53FB84E: eglCreateContext (eglapi.c:821)
 ==45872==
 ==45872== This conflicts with a previous read of size 8 by thread #544
 ==45872== Locks held: 1, at address 0x5F6E0E0
 ==45872==    at 0x6CB779E: blorp_alloc_binding_table (iris_blorp.c:167)
 ==45872==    by 0x6CAEF70: blorp_emit_surface_states (blorp_genX_exec.h:1540)
 ==45872==    by 0x6CB67F9: blorp_exec (blorp_genX_exec.h:2016)
 ==45872==    by 0x6CB7AFE: iris_blorp_exec (iris_blorp.c:307)
 ==45872==    by 0x70F5916: try_blorp_blit (blorp_blit.c:2145)
 ==45872==    by 0x70F5FCA: do_blorp_blit (blorp_blit.c:2273)
 ==45872==    by 0x70F778F: blorp_copy (blorp_blit.c:2803)
 ==45872==    by 0x6BB9EB6: iris_copy_region (iris_blit.c:725)

v2: move as genX(init_screen_state) (Lionel)

Signed-off-by: Tapani Pälli <tapani.palli@intel.com>
Reviewed-by: Lionel Landwerlin <lionel.g.landwerlin@intel.com>
Cc: mesa-stable
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/7544>