etnaviv: linear PE support

Add support for rendering into linear buffers on GPUs that have that capability. This is quite helpful on new variants of the i.MX8M*, which only have very simple scanout engines that aren't able to read GPU tiled buffers, but GPUs with the LINEAR_PE feature. Allows to skip a fullscreen blit to convert between GPU tiled shadow and the actual scanout buffer. As linear rendering is slower than rendering into tiled, never actively chose the linear buffer layout, only use it when importing a linear buffer or being forced to linear via a usage flag or modifier on allocation.