msm: mdss: change error return value for lineptr sysfs entries

Changing the error return value for lineptr sysfs entries, when
the sysfs operation is not allowed. Returning EAGAIN might make
the verification tests to continuously query the sysfs nodes, or
block it. Returning EPERM will fail the query, which is expected.

Change-Id: I43500c07a318da9e2624a1631134228ad45e1cc7
Signed-off-by: Krishna Chaitanya Devarakonda <kdevarak@codeaurora.org>