disp: msm: dsi: fix boot display name parsing

The boot_disp_en flag for secondary display is getting enabled
all the time, even if its boot_param string is empty.
Correct the boot display name parsing from boot_param string
to fix this issue.

Change-Id: Ica1465611f592b22518061987571919838914891
Signed-off-by: Rajeev Nandan <rajeevny@codeaurora.org>