drm/sti: remove redundant sign extensions

arg is long int, so arg = (arg << 22) >> 22 makes the upper 22 bits of
arg equal to bit 9 (or bit 41). But we then mask away all but bits 0-9, so
this is entirely redundant.

Signed-off-by: Rasmus Villemoes <linux@rasmusvillemoes.dk>
Reviewed-by: Vincent Abriou <vincent.abriou@st.com>