drm/amdgpu/gfx: create a common bitmask function (v2)

The same function was duplicated in all the gfx IPs. Use
a single implementation for all.

v2: use static inline (Alex Xie)

Reviewed-by: Alex Xie <AlexBin.Xie@amd.com>
Suggested-by: Andres Rodriguez <andresx7@gmail.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>