mesa: Add support to unpack depth-stencil texture in to FLOAT_32_UNSIGNED_INT_24_8_REV

mesa: Add support to unpack depth-stencil texture in to FLOAT_32_UNSIGNED_INT_24_8_REV

V2: Follow the new naming convention for unpack functions.
    Use double precision for converting Z24 to a float.
V3: Unpack stencil value to most significant byte.
    Use 'struct z32f_x24s8' type.
V4: Unpack stencil value to least significant byte.
    Add a comment to clarify stencil packing.

Cc: <mesa-stable@lists.freedesktop.org>
Signed-off-by: Anuj Phogat <anuj.phogat@gmail.com>
Reviewed-by: Brian Paul <brianp@vmware.com>
Reviewed-by: Ian Romanick <ian.d.romanick@intel.com>