drm/amd/display: Move backlight pwm enable function call

[Why]
Can't call dmub_abm_enable_fractional_pwm from dmub_abm_create as
dmub_srv is still null at this init stage, and therefore can't call to
fw.

[How]
Move call to dmub_abm_init_backlight. This should be the first call from
the driver for ABM.

Signed-off-by: Wyatt Wood <wyatt.wood@amd.com>
Reviewed-by: Nicholas Kazlauskas <Nicholas.Kazlauskas@amd.com>
Acked-by: Rodrigo Siqueira <Rodrigo.Siqueira@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>