i965: add missing `const` in function signature

i965: add missing `const` in function signature

Gets rid of a few warnings of the form:
  src/mesa/drivers/dri/i965/intel_screen.c:918:49: warning: passing argument 2 of ‘modifier_is_supported’ discards ‘const’ qualifier from pointer target type [-Wdiscarded-qualifiers]
          !modifier_is_supported(&screen->devinfo, f, 0, modifier))
                                                   ^
  src/mesa/drivers/dri/i965/intel_screen.c:301:1: note: expected ‘struct intel_image_format *’ but argument is of type ‘const struct intel_image_format *’

Fixes: 1efd73df39b39589d26f "i965: Advertise the CCS modifier"
Cc: Ben Widawsky <ben@bwidawsk.net>
Signed-off-by: Eric Engestrom <eric.engestrom@imgtec.com>
Reviewed-by: Matt Turner <mattst88@gmail.com>