drm/amd: Use dynamic array size declaration for amdgpu_connector_add_common_modes()

drm/amd: Use dynamic array size declaration for amdgpu_connector_add_common_modes()

[Why]
Adding or removing a mode from common_modes[] can be fragile if a user
forgot to update the for loop boundaries.

[How]
Use ARRAY_SIZE() to detect size of the array and use that instead.

Cc: Timur Kristóf <timur.kristof@gmail.com>
Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Reviewed-by: Timur Kristóf <timur.kristof@gmail.com>
Link: https://lore.kernel.org/r/20250924161624.1975819-4-mario.limonciello@amd.com
Signed-off-by: Mario Limonciello <mario.limonciello@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>