drm/gma500: remove unused gma_pipe_event function

clang with W=1 reports
drivers/gpu/drm/gma500/psb_irq.c:35:19: error: unused function
  'gma_pipe_event' [-Werror,-Wunused-function]
static inline u32 gma_pipe_event(int pipe)
                  ^
This function is not used, so remove it.

Signed-off-by: Tom Rix <trix@redhat.com>
Signed-off-by: Patrik Jakobsson <patrik.r.jakobsson@gmail.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20230319142320.1704336-1-trix@redhat.com