display: Make diaplay lock data private

display: Make diaplay lock data private

Changed list name 'cond_head' to 'g_display_lock_list'. Previously, the
list had been exposed to another module using get_cond_head(). Remove
the function to protect the data from being accessed oustide of the
display-lock.c. Instead, the display-lock.c now provides services that
the other module has done using get_cond_head(). Currently, the only
below function has been added for display-misc.c.
 - display_lock_print_lock_info()

All existing get_cond_head() has been replaced by proper functions.

Change-Id: I4e4603f44c6613d71bb0b1e05abe6d45ae4a39c5
Signed-off-by: Youngjae Cho <y0.cho@samsung.com>