exynos/fimg2d: make g2d_add_cmd() less heavy

The function currently checks for each added command
if an overflow of the corresponding command buffers
occurs, but none of the callers ever checks the
return value.

Since all callers are now converted to use
g2d_check_space() simplify the function.

(1) The overflow checks become asserts, so they're only
    active for debug builds. This is fine since
    g2d_add_cmd() is not part of the public API.

(2) Switch the return value to void.

(3) Explicitly state that the caller has to check
    buffer space before calling g2d_add_cmd().

Signed-off-by: Tobias Jakobi <tjakobi@math.uni-bielefeld.de>
Reviewed-by: Emil Velikov <emil.l.velikov@gmail.com>