While the KMS/DRM support came fairly quickly for Southern Islands to at least provide kernel mode-setting support, the Gallium3D driver for providing OpenGL has been very slow to mature. With the HD 7000 series, the 2D acceleration in xf86-video-ati is also being done using the GLAMOR acceleration architecture, which puts OpenGL as a prerequisite for getting 2D acceleration.
Fortunately, there's been a number of RadeonSI driver patches hitting the dri-devel mailing list this week:
radeonsi: Pass texture type to sampling intrinsics
radeonsi: Consolidate calculation of tile mode index
radeonsi: Enable tiling for depth/stencil resources
radeonsi: Use proper hardware format for stencil texturing
radeonsi: Set SPI_SHADER_COL_FORMAT to what the pixel shader actually exports
radeonsi: Fix Z24 texture formats
radeonsi: Actually keep track if we are using depth textures for samplers
radeonsi/vdpau: remove nonsense state tracker dep
Until the RadeonSI Gallium3D driver is in better shape, at least there is the AMD Catalyst driver that's been providing a working binary blob that's feature-complete since day one. If you missed it from yesterday, also checkout The State Of Open-Source Radeon Driver Features.