iris: Allow max dynamic pool size of 2GB for gen12
Some piglit tests that fail without this change:
fbo-draw-buffers-blend -auto -fbo
fbo-drawbuffers2-colormask -auto -fbo
bgra-sec-color-pointer -auto
fbo-alphatest-formats GL_ARB_texture_float -auto -fbo
fbo-blending-formats GL_ARB_texture_rg -auto -fbo
fbo-colormask-formats GL_EXT_texture_snorm -auto -fbo
fbo-blending-formats GL_ARB_texture_rg-float -auto -fbo
ext_framebuffer_multisample-alpha-blending-after-rendering 2 -auto -fbo
Cc: @kwg