drm/amd/display: set backlight level limit to 1

This patch will work as workaround for silicon limitation
related to PWM dutycycle when the backlight level goes to 0.

Actually PWM value is 16 bit value and valid range from 1-65535.
when ever user requested to set this PWM value to 0 which is not
fall in the range, in VBIOS taken care this by limiting to 1.
This patch here will do the same. Either driver or VBIOS can not
pass 0 value as it is not a valid range for PWM and it will
give a high PWM pulse which is not the intended behaviour as
per HW constraints.

Signed-off-by: suresh guttula <suresh.guttula@amd.com>
Reviewed-by: Harry Wentland <harry.wentland@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>