Move backlight_read and backlight_write functions from
kms_pm_backlight to library to reuse in other tests.
Move and rename struct context to intel_backlight_context_t.

v2: Rename backlight_read to igt_backlight_read and backlight_write to
    backlight_write (Suraj).
    Add a new member to intel_backlight_context_t to cache vendor specific
    backlight path.

Signed-off-by: Santhosh Reddy Guddati <santhosh.reddy.guddati@intel.com>
Reviewed-by: Suraj Kandpal <suraj.kandpal@intel.com>