broadcom/compiler: fix first_component assertion

first_component is an uint, and thus if it takes value 0 we can't know
if it is because writemask has its first bit to 1, or all bits to 0.

As we want to ensure that at least one bit is set, apply the assertion
in writemask.

Fixes CID#1472829 "Macro compares unsigned to 0 (NO_EFFECT)".

v2:
 - Restore "first_component <= last_component" assertion (Iago)

Reviewed-by: Iago Toral Quiroga <itoral@igalia.com>
Signed-off-by: Juan A. Suarez Romero <jasuarez@igalia.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/10103>