panfrost: Don't lose bits!

panfrost: Don't lose bits!

UBSAN complained that when alpha was 255 and we shifted it 24 positions
to the left, it didn't fit in a signed int. That's because bitwise
operations automatically promote to signed int.

../src/gallium/drivers/panfrost/pan_job.c:1130:64: runtime error: left shift of 255 by 24 places cannot be represented in type 'int'"}
    #0 0xacf953d6 in pan_pack_color ../src/gallium/drivers/panfrost/pan_job.c:1130"}
    #1 0xacf953d6 in panfrost_batch_clear ../src/gallium/drivers/panfrost/pan_job.c:1204"}
    #2 0xaae3226a in st_Clear ../src/mesa/state_tracker/st_cb_clear.c:513"}
    #3 0x4c3d0e in deqp::gles2::TestCaseWrapper::iterate(tcu::TestCase*) (/deqp/modules/gles2/deqp-gles2+0x2ad0e)"}
    #4 0x828cf2 in tcu::TestSessionExecutor::iterateTestCase(tcu::TestCase*) (/deqp/modules/gles2/deqp-gles2+0x38fcf2)"}
    #5 0x8295f0 in tcu::TestSessionExecutor::iterate() (/deqp/modules/gles2/deqp-gles2+0x3905f0)"}
    #6 0x810aac in tcu::App::iterate() (/deqp/modules/gles2/deqp-gles2+0x377aac)"}
    #7 0x4c1d4c in main (/deqp/modules/gles2/deqp-gles2+0x28d4c)"}
    #8 0xb64b6aa8 in __libc_start_main (/lib/arm-linux-gnueabihf/libc.so.6+0x1aaa8)"}

Signed-off-by: Tomeu Vizoso <tomeu.vizoso@collabora.com>
Reviewed-by: Alyssa Rosenzweig <alyssa.rosenzweig@collabora.com>