st/mesa: use enum types instead of int/unsigned (v3)

st/mesa: use enum types instead of int/unsigned (v3)

Use the proper enum types for various variables.  Makes life in gdb
a little nicer.  Note that the size of enum bitfields must be one
larger so the high bit is always zero (for MSVC).

v2: also increase size of image_format bitfield, per Eric Engestrom.
v3: use the new ASSERT_BITFIELD_SIZE() macro

Reviewed-by: Charmaine Lee <charmainel@vmware.com>
Reviewed-by: Roland Scheidegger <sroland@vmware.com>
Reviewed-by: Nicolai Hähnle <nicolai.haehnle@amd.com>