gallium, iris: Fix misrendering among EGLImage siblings
Disable compression in iris_flush_resource if the resource lacks a modifier. When a caller wants to prepare such a resource for sharing (via eglCreateImage for example), this change enables all reference holders to access the resource in a common manner - without compression.
This fixes misrendering with 3D-accelerated qemu. A piglit test which reproduces qemu's behavior, ext_image_dma_buf_import-export-tex, is also enabled to pass.
Closes: #2678 (closed)