drm/i915: Skip .get_backlight() when backlight isn't enabled
drm/i915: Skip .get_backlight() when backlight isn't enabled

On VLV/CHV when the display is off, we can't read out the current
backlight level from the hardware since we have no pipe to do so.
Currently we end up reading a bigus register due to passing
INVALID_PIPE to VLV_BLC_PWM_CTL().

Skip the entire .get_backlight() call if the backlight isn't enabled
according to backlight.enabled.

This problem can be reproduced simply by reading the backlight device
actual_brightness file while the display is off.

Cc: Jani Nikula <ba783f3beccaedfda693f41a15407d612a629408@intel.com>
Suggested-by: Jani Nikula <ba783f3beccaedfda693f41a15407d612a629408@intel.com>
Signed-off-by: Ville Syrjälä <cd6e8d405ca90be3a03d5427c5b24fbd2d68dcc4@linux.intel.com>
Reviewed-by: Jani Nikula <ba783f3beccaedfda693f41a15407d612a629408@intel.com>
Signed-off-by: Daniel Vetter <c1b6782c4af8f0673da8923a0702a1832e5940f4@ffwll.ch>

KristFoundation/Programs,KristFoundation/Programs,KristFoundation/Programs,KristFoundation/Programs,KristFoundation/Programs,KristFoundation/Programs