drivers: gpu: amd: Initialize amdgpu_dm_backlight_caps object to 0 in amdgpu_dm_updat...

drivers: gpu: amd: Initialize amdgpu_dm_backlight_caps object to 0 in amdgpu_dm_update_backlight_caps

In `amdgpu_dm_update_backlight_caps()`, there is a local
`amdgpu_dm_backlight_caps` object that is filled in by
`amdgpu_acpi_get_backlight_caps()`. However, this object is
uninitialized before the call and hence the subsequent check for
aux_support can fail since it is not initialized by
`amdgpu_acpi_get_backlight_caps()` as well. This change initializes
this local `amdgpu_dm_backlight_caps` object to 0.

Reviewed-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Furquan Shaikh <furquan@google.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>