drm/amd/display: Remove duplicate code across dcn30 and dcn31

The function CalculateBytePerPixelAnd256BBlockSizes was defined four
times: on display_mode_vba_30.c, display_rq_dlg_calc_30.c,
display_mode_vba_31.c and display_rq_dlg_calc_31.c. In order to avoid
code duplication, the CalculateBytePerPixelAnd256BBlockSizes is defined
on display_mode_vba_30.h and used across dcn30 and dcn31.

Signed-off-by: Maíra Canal <mairacanal@riseup.net>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>