drm/amd: Query and use ACPI backlight caps

ACPI ATIF has a function called query
backlight transfer characteristics.  Among the
information returned by this function is
the minimum and maximum input signals for the
backlight

Call that function on ACPI init.  When DM
backlight device is updated, copy over the
backlight caps into DM, but only once.  Use
the backlight caps in the backlight-to-dc
calculation

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

drivers/gpu/drm/amd/amdgpu/amdgpu.h

@@ -1252,6 +1252,9 @@ bool amdgpu_acpi_is_pcie_performance_request_supported(struct amdgpu_device *ade
 int amdgpu_acpi_pcie_performance_request(struct amdgpu_device *adev,
 						u8 perf_req, bool advertise);
 int amdgpu_acpi_pcie_notify_device_ready(struct amdgpu_device *adev);
+
+void amdgpu_acpi_get_backlight_caps(struct amdgpu_device *adev,
+		struct amdgpu_dm_backlight_caps *caps);
 #else
 static inline int amdgpu_acpi_init(struct amdgpu_device *adev) { return 0; }
 static inline void amdgpu_acpi_fini(struct amdgpu_device *adev) { }