Not being a programmer or EE I may have missed this, but it was my understanding that these graphics chips in mobile chips are one-off designs. For example, although the PVR core maybe similar every chip that includes the core has differences, and with each generation these chips change significantly. So even though there may only really be 3-4 ARM designs and 1-2 Graphics cores, combined every iteration is really a separate and distinct version with it's own bugs and kinks.
If the following is true, what is the point in ever merging any driver for these devices into mainline? If the driver is a one-off, has to be revised for every revision or version produced there could end up being a LOT of drivers that are all different. Factor out a decade and you could end up with a thousand different drivers for one-off designs that are abandoned/revised 6months-yearly.
I see no point in merging drivers for devices that have no stability, longevity or persistence. At least the Intel/ATI/Nvidia hardware is a consistent design for a period of time with many more sales per design and often a single design with multiple products. Some of these mobile designs could be used in only a few products and end up with little to no market share to where you are dealing with a design used by 0.00001% of the global population. The fear of bit-rot on these drivers is very well founded IMO.
Unless the manufactures can come together and promise to maintain an interface for a period of time where ALL products (or at least >50%) in the class use the same interface there is little purpose in merging IMO.