media: atomisp: get rid of atomisp_get_frame_pgnr() abstraction

Simplify the code by getting rid of this function, as it ends
being just a single line of code.

Link: https://lore.kernel.org/linux-media/80ea920d14379124ba92aab2e6a6d12a92d79b2b.1636544620.git.mchehab+huawei@kernel.org


Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>