drm/msm/dp: rename is_connected with link_ready

The is_connected flag is set to true after DP mainlink successfully
finishes link training to enter into ST_MAINLINK_READY state rather
than being set after the DP dongle is connected. Rename the
is_connected flag with link_ready flag to match the state of DP
driver's state machine.

Changes in v5:
-- reworded commit text according to review comments from change #4

Changes in v4:
-- reworded commit text

Signed-off-by: Kuogee Hsieh <quic_khsieh@quicinc.com>
Reviewed-by: Dmitry Baryshkov <dmitry.baryshkov@linaro.org>
Patchwork: https://patchwork.freedesktop.org/patch/570063/
Link: https://lore.kernel.org/r/1701472789-25951-3-git-send-email-quic_khsieh@quicinc.com
Signed-off-by: Dmitry Baryshkov <dmitry.baryshkov@linaro.org>