Last year AMD marked the Iceland support as experimental due to not having tested the new GPUs much and some bugs were coming in regarding the AMDGPU DRM driver's support for the mobile GPUs.
Fast forward to today, Alex Deucher posted a series of eight AMDGPU patches for Iceland/Topaz and at the end they remove the experimental support flag. Thus the hardware-accelerated open-source support will now be available without the amdgpu.exp_hw_support=1 kernel module parameter. The fixes done prior to re-enabling the out-of-the-box graphics support appear to be mainly related to the firmware handling for this family of graphics processors.
Unless this patch series gets sent in today as "fixes" for Linux 4.5, this experimental support flag won't be removed until Linux 4.6.
Iceland/Topaz is found in the Radeon R5 M255 and R7 M260 mobile graphics processors. The PCI IDs listed by this family of GPUs are 0x6900, 0x6901, 0x6902, 0x6903, and 0x6907.