st/mesa: Add rgbx handling for fp formats

Add missing cases for fp32 and fp16 formats.

Fixes: c68334ff "st/mesa: add floating point formats in st_new_renderbuffer_fb()"
Signed-off-by: Kevin Strasser <kevin.strasser@intel.com>
Reviewed-by: Emil Velikov <emil.velikov@collabora.com>
Reviewed-by: Eric Engestrom <eric.engestrom@intel.com>
(cherry picked from commit 845ec857)