remove BAGL-gating on screen syscalls declaration

[yhql]

Description

Screen drawing syscalls like bagl_hal_draw_bitmap_within_rect, ... were gated by HAVE_BAGL and HAVE_SE_SCREEN in os_screen.h whereas the syscalls implementation in syscalls.c only use HAVE_SE_SCREEN.
HAVE_BAGL was removed as the syscalls are there whether we use this define or not, and also the set_brightness syscall was gated to match syscalls.c.

Changes include

• Other (for changes that might not fit in any category)