i965: Set MaxFramebufferWidth/Height to 16384, not viewport.

i965: Set MaxFramebufferWidth/Height to 16384, not viewport.

dEQP-GLES31.functional.fbo.no_attachments.maximums.{all,height,size,width}
started hitting assertion failures when emitting SURFACE_STATE, after
commit e8fd60e7891c7 where Samuel increased the maximum viewport size to
32768, from 16384.

MaxFramebufferWidth/Height were being set to the maximum viewport size,
but are actually limited by the SURFACE_STATE width/height field range,
which is 16384 on Gen7+ (where ARB_framebuffer_no_attachments is
exposed).  So, reduce these to 16384 explicitly.

Fixes assert fails in the above mentioned dEQP tests.  (Those tests
still fail, however.)

Signed-off-by: Kenneth Graunke <kenneth@whitecape.org>
Reviewed-by: Samuel Iglesias Gonsálvez <siglesias@igalia.com>