drm: etnaviv: extract command ring reservation

Provide a helper etnaviv_buffer_reserve() to ensure that we can fit a
set of commands into the ring buffer without wrapping by moving code
out of etnaviv_buffer_queue().

Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>
Signed-off-by: Lucas Stach <l.stach@pengutronix.de>