drm: panel-backlight-quirks: Convert brightness quirk to generic structure

drm: panel-backlight-quirks: Convert brightness quirk to generic structure

Currently, the brightness quirk is limited to minimum brightness only.
Refactor it to a structure, so that more quirks can be added in the
future. Reserve 0 value for "no quirk", and use u16 to allow minimum
brightness up to 255.

Tested-by: Philip Müller <philm@manjaro.org>
Reviewed-by: Mario Limonciello <mario.limonciello@amd.com>
Signed-off-by: Antheas Kapenekakis <lkml@antheas.dev>
Link: https://lore.kernel.org/r/20250829145541.512671-3-lkml@antheas.dev
Acked-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Mario Limonciello (AMD) <superm1@kernel.org>