backend-drm: use pixel_format_info instead of gbm_format and drop switch case in drm_output_init_pixman()

Use struct pixel_format_info pointers instead of uint32_t gbm_format values in struct drm_backend and struct drm_output.

Potentially allow more pixel formats by removing the switch case in drm_output_init_pixman().

Based on !1130 (merged). Suggested in !1130 (comment 1740587) and !1130 (comment 1742558).