mesa/main/readpix: Correct handling of packed floating point values

mesa/main/readpix: Correct handling of packed floating point values

Make sure that clamping in the pixel transfer operations is enabled/disabled
for packed floating point values just like it is done for single normal and
half precision floating point values.

This fixes a series of CTS tests with virgl that use r11f_g11f_b10f
buffers as target, and where virglrenderer reads these surfaces back
using the format GL_UNSIGNED_INT_10F_11F_11F_REV.

Signed-off-by: Gert Wollny <gert.wollny@collabora.com>
Reviewed-by: Gurchetan Singh <gurchetansingh@chromium.org>
Reviewed-by: Ian Romanick <ian.d.romanick@intel.com>