drm/gma500: Cosmetic cleanup of irq code

Use the gma_ prefix instead of psb_ since the code is common for all
chips. Various coding style fixes. Removal of unused code. Removal of
duplicate function declarations.

Signed-off-by: Patrik Jakobsson <patrik.r.jakobsson@gmail.com>
Acked-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: https://patchwork.freedesktop.org/patch/msgid/20220317092555.17882-4-patrik.r.jakobsson@gmail.com