drm/bridge: ti-sn65dsi86: Check for CONFIG_PWM using IS_REACHABLE()

Currently CONFIG_PWM is a bool but I intend to change it to tristate. If
CONFIG_PWM=m in the configuration, the cpp symbol CONFIG_PWM isn't
defined and so the PWM code paths in the ti-sn65dsi86 driver are not
used.

The correct way to check for CONFIG_PWM is using IS_REACHABLE which does
the right thing for all cases
CONFIG_DRM_TI_SN65DSI86 ∈ { y, m } x CONFIG_PWM ∈ { y, m, n }.

There is no change until CONFIG_PWM actually becomes tristate.

Signed-off-by: Uwe Kleine-König <u.kleine-koenig@baylibre.com>
Reviewed-by: Louis Chauvet <louis.chauvet@bootlin.com>
Reviewed-by: Robert Foss <rfoss@kernel.org>
Reviewed-by: Douglas Anderson <dianders@chromium.org>
Signed-off-by: Douglas Anderson <dianders@chromium.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20250217174936.758420-2-u.kleine-koenig@baylibre.com