drm/amd/display: Restore guard against default backlight value < 1 nit

Mark reports that brightness is not restored after Xorg dpms screen blank.

This behavior was introduced by commit d9e86582 ("drm/amd/display:
Simplify brightness initialization") which dropped the cached backlight
value in display code, but also removed code for when the default value
read back was less than 1 nit.

Restore this code so that the backlight brightness is restored to the
correct default value in this circumstance.

Reported-by: Mark Herbert <mark.herbert42@gmail.com>
Closes: drm/amd#3031


Cc: stable@vger.kernel.org
Cc: Camille Cho <camille.cho@amd.com>
Cc: Krunoslav Kovac <krunoslav.kovac@amd.com>
Cc: Hamza Mahfooz <hamza.mahfooz@amd.com>
Fixes: d9e86582 ("drm/amd/display: Simplify brightness initialization")
Acked-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Mario Limonciello <mario.limonciello@amd.com>