UNORM Depth buffer precision reduced to 22 or 15 bits

[brianwatling]

Precision was reduced to 22 bits for 24 bit UNORM depth buffers and to 15 for 16 bit UNORM depth buffers in 6fa4f8a (see changes in src/core/hw/gfxip/gfx9/gfx9DepthStencilView.cpp or src/core/hw/gfxip/gfx6/gfx6DepthStencilView.cpp). The change was introduced to correct rounding issues when depth bias is 1.0f, but reduces image quality overall. A solution what fixes the rounding issue while maintaining precision would be ideal