gallium: separate out floating-point CAPs into its own enum

gallium: separate out floating-point CAPs into its own enum

The motivation behind this is to add some self-documentation in the code
about how each CAP can be used.

The idea is:
- enum pipe_cap is only valid in get_param
- enum pipe_capf is only valid in get_paramf

Which CAPs are floating-point have been determined based on how everybody
except svga implemented the functions. svga have been modified to match all
the other drivers.

Besides that, the floating-point CAPs are now prefixed with PIPE_CAPF_.