drm: meson: mask value when writing bits relaxed

The value used in the macro writel_bits_relaxed has to be masked since
we don't want change the bits outside the mask.

Signed-off-by: Julien Masson <jmasson@baylibre.com>
Reviewed-by: Neil Armstrong <narmstrong@baylibre.com>
Signed-off-by: Neil Armstrong <narmstrong@baylibre.com>
Link: https://patchwork.freedesktop.org/patch/msgid/86y31r82fo.fsf@baylibre.com