input: misc: hbtp_input: allow read access to display_pwr sysfs

Add show function to allow read access to display_pwr sysfs
entry. Add removal of sysfs and kobject in hbtp_exit.

Change-Id: I2b1576f93a45ea4e4016a71fc43fc4256816b2f7
Signed-off-by: Alex Sarraf <asarraf@codeaurora.org>